diff --git a/src/app/components/AccountDataEditor.tsx b/src/app/components/AccountDataEditor.tsx
index 2dbaf1f1f..76b73dda2 100644
--- a/src/app/components/AccountDataEditor.tsx
+++ b/src/app/components/AccountDataEditor.tsx
@@ -294,7 +294,7 @@ export function AccountDataEditor({
             </Chip>
           </Box>
           <Box shrink="No">
-            <IconButton onClick={requestClose} variant="Surface">
+            <IconButton onClick={requestClose} variant="Surface" aria-label="Close">
               <Icon src={Icons.Cross} />
             </IconButton>
           </Box>
diff --git a/src/app/components/DeviceVerification.tsx b/src/app/components/DeviceVerification.tsx
index d2502ccf7..53875f9fc 100644
--- a/src/app/components/DeviceVerification.tsx
+++ b/src/app/components/DeviceVerification.tsx
@@ -261,7 +261,7 @@ export function DeviceVerification({ request, onExit }: DeviceVerificationProps)
               <Box grow="Yes">
                 <Text size="H4">Device Verification</Text>
               </Box>
-              <IconButton size="300" radii="300" onClick={handleCancel}>
+              <IconButton size="300" radii="300" onClick={handleCancel} aria-label="Cancel verification">
                 <Icon src={Icons.Cross} />
               </IconButton>
             </Header>
diff --git a/src/app/components/DeviceVerificationSetup.tsx b/src/app/components/DeviceVerificationSetup.tsx
index 433fa6a1d..1e13e2f97 100644
--- a/src/app/components/DeviceVerificationSetup.tsx
+++ b/src/app/components/DeviceVerificationSetup.tsx
@@ -301,7 +301,7 @@ export const DeviceVerificationSetup = forwardRef<HTMLDivElement, DeviceVerifica
           <Box grow="Yes">
             <Text size="H4">Setup Device Verification</Text>
           </Box>
-          <IconButton size="300" radii="300" onClick={onCancel}>
+          <IconButton size="300" radii="300" onClick={onCancel} aria-label="Cancel">
             <Icon src={Icons.Cross} />
           </IconButton>
         </Header>
diff --git a/src/app/components/editor/Toolbar.tsx b/src/app/components/editor/Toolbar.tsx
index 7d701c424..b6be7b878 100644
--- a/src/app/components/editor/Toolbar.tsx
+++ b/src/app/components/editor/Toolbar.tsx
@@ -54,8 +54,8 @@ function BtnTooltip({ text, shortCode }: { text: string; shortCode?: string }) {
   );
 }
 
-type MarkButtonProps = { format: MarkType; icon: IconSrc; tooltip: ReactNode };
-export function MarkButton({ format, icon, tooltip }: MarkButtonProps) {
+type MarkButtonProps = { format: MarkType; icon: IconSrc; tooltip: ReactNode; label?: string };
+export function MarkButton({ format, icon, tooltip, label }: MarkButtonProps) {
   const editor = useSlate();
   const disableInline = isBlockActive(editor, BlockType.CodeBlock);
 
@@ -76,6 +76,7 @@ export function MarkButton({ format, icon, tooltip }: MarkButtonProps) {
           variant="SurfaceVariant"
           onClick={handleClick}
           aria-pressed={isMarkActive(editor, format)}
+          aria-label={label}
           size="400"
           radii="300"
           disabled={disableInline}
@@ -91,8 +92,9 @@ type BlockButtonProps = {
   format: BlockType;
   icon: IconSrc;
   tooltip: ReactNode;
+  label?: string;
 };
-export function BlockButton({ format, icon, tooltip }: BlockButtonProps) {
+export function BlockButton({ format, icon, tooltip, label }: BlockButtonProps) {
   const editor = useSlate();
 
   const handleClick = () => {
@@ -108,6 +110,7 @@ export function BlockButton({ format, icon, tooltip }: BlockButtonProps) {
           variant="SurfaceVariant"
           onClick={handleClick}
           aria-pressed={isBlockActive(editor, format)}
+          aria-label={label}
           size="400"
           radii="300"
         >
@@ -165,6 +168,7 @@ export function HeadingBlockButton() {
                   <IconButton
                     ref={triggerRef}
                     onClick={() => handleMenuSelect(1)}
+                    aria-label="Heading 1"
                     size="400"
                     radii="300"
                   >
@@ -180,6 +184,7 @@ export function HeadingBlockButton() {
                   <IconButton
                     ref={triggerRef}
                     onClick={() => handleMenuSelect(2)}
+                    aria-label="Heading 2"
                     size="400"
                     radii="300"
                   >
@@ -195,6 +200,7 @@ export function HeadingBlockButton() {
                   <IconButton
                     ref={triggerRef}
                     onClick={() => handleMenuSelect(3)}
+                    aria-label="Heading 3"
                     size="400"
                     radii="300"
                   >
@@ -271,32 +277,44 @@ export function Toolbar() {
               <MarkButton
                 format={MarkType.Bold}
                 icon={Icons.Bold}
-                tooltip={<BtnTooltip text="Bold" shortCode={`${modKey} + B`} />}
+                tooltip={<BtnTooltip text="Bold" shortCode={`${modKey} + B`}
+                label="Bold"
+              />}
               />
               <MarkButton
                 format={MarkType.Italic}
                 icon={Icons.Italic}
-                tooltip={<BtnTooltip text="Italic" shortCode={`${modKey} + I`} />}
+                tooltip={<BtnTooltip text="Italic" shortCode={`${modKey} + I`}
+                label="Italic"
+              />}
               />
               <MarkButton
                 format={MarkType.Underline}
                 icon={Icons.Underline}
-                tooltip={<BtnTooltip text="Underline" shortCode={`${modKey} + U`} />}
+                tooltip={<BtnTooltip text="Underline" shortCode={`${modKey} + U`}
+                label="Underline"
+              />}
               />
               <MarkButton
                 format={MarkType.StrikeThrough}
                 icon={Icons.Strike}
-                tooltip={<BtnTooltip text="Strike Through" shortCode={`${modKey} + S`} />}
+                tooltip={<BtnTooltip text="Strike Through" shortCode={`${modKey} + S`}
+                label="Strikethrough"
+              />}
               />
               <MarkButton
                 format={MarkType.Code}
                 icon={Icons.Code}
-                tooltip={<BtnTooltip text="Inline Code" shortCode={`${modKey} + [`} />}
+                tooltip={<BtnTooltip text="Inline Code" shortCode={`${modKey} + [`}
+                label="Inline code"
+              />}
               />
               <MarkButton
                 format={MarkType.Spoiler}
                 icon={Icons.EyeBlind}
-                tooltip={<BtnTooltip text="Spoiler" shortCode={`${modKey} + H`} />}
+                tooltip={<BtnTooltip text="Spoiler" shortCode={`${modKey} + H`}
+                label="Spoiler"
+              />}
               />
             </Box>
             <Line variant="SurfaceVariant" direction="Vertical" style={{ height: toRem(12) }} />
@@ -305,22 +323,30 @@ export function Toolbar() {
             <BlockButton
               format={BlockType.BlockQuote}
               icon={Icons.BlockQuote}
-              tooltip={<BtnTooltip text="Block Quote" shortCode={`${modKey} + '`} />}
+              tooltip={<BtnTooltip text="Block Quote" shortCode={`${modKey} + '`}
+              label="Block quote"
+            />}
             />
             <BlockButton
               format={BlockType.CodeBlock}
               icon={Icons.BlockCode}
-              tooltip={<BtnTooltip text="Block Code" shortCode={`${modKey} + ;`} />}
+              tooltip={<BtnTooltip text="Block Code" shortCode={`${modKey} + ;`}
+              label="Code block"
+            />}
             />
             <BlockButton
               format={BlockType.OrderedList}
               icon={Icons.OrderList}
-              tooltip={<BtnTooltip text="Ordered List" shortCode={`${modKey} + 7`} />}
+              tooltip={<BtnTooltip text="Ordered List" shortCode={`${modKey} + 7`}
+              label="Ordered list"
+            />}
             />
             <BlockButton
               format={BlockType.UnorderedList}
               icon={Icons.UnorderList}
-              tooltip={<BtnTooltip text="Unordered List" shortCode={`${modKey} + 8`} />}
+              tooltip={<BtnTooltip text="Unordered List" shortCode={`${modKey} + 8`}
+              label="Unordered list"
+            />}
             />
             <HeadingBlockButton />
           </Box>
diff --git a/src/app/components/event-readers/EventReaders.tsx b/src/app/components/event-readers/EventReaders.tsx
index 14d36fb28..58b1f56ea 100644
--- a/src/app/components/event-readers/EventReaders.tsx
+++ b/src/app/components/event-readers/EventReaders.tsx
@@ -73,7 +73,7 @@ export const EventReaders = as<'div', EventReadersProps>(
           <Box grow="Yes">
             <Text size="H3" style={lotusTerminal ? { color: '#00D4FF', textShadow: '0 0 6px rgba(0,212,255,0.45)', letterSpacing: '0.05em' } : undefined}>Seen by</Text>
           </Box>
-          <IconButton size="300" onClick={requestClose}>
+          <IconButton size="300" onClick={requestClose} aria-label="Close">
             <Icon src={Icons.Cross} />
           </IconButton>
         </Header>
diff --git a/src/app/components/password-input/PasswordInput.tsx b/src/app/components/password-input/PasswordInput.tsx
index 4ffe0d831..3e1e278d7 100644
--- a/src/app/components/password-input/PasswordInput.tsx
+++ b/src/app/components/password-input/PasswordInput.tsx
@@ -28,6 +28,7 @@ export const PasswordInput = forwardRef<HTMLInputElement, PasswordInputProps>(
                   variant={visible ? 'Warning' : variant}
                   size="300"
                   radii="300"
+                  aria-label={visible ? 'Hide password' : 'Show password'}
                 >
                   <Icon
                     style={{ opacity: config.opacity.P300 }}
diff --git a/src/app/components/uia-stages/PasswordStage.tsx b/src/app/components/uia-stages/PasswordStage.tsx
index 0164cae6f..41954a6cf 100644
--- a/src/app/components/uia-stages/PasswordStage.tsx
+++ b/src/app/components/uia-stages/PasswordStage.tsx
@@ -45,7 +45,7 @@ export function PasswordStage({
         <Box grow="Yes">
           <Text size="H4">Account Password</Text>
         </Box>
-        <IconButton size="300" onClick={onCancel} radii="300">
+        <IconButton size="300" onClick={onCancel} radii="300" aria-label="Cancel">
           <Icon src={Icons.Cross} />
         </IconButton>
       </Header>
diff --git a/src/app/components/uia-stages/SSOStage.tsx b/src/app/components/uia-stages/SSOStage.tsx
index e3d6126a5..2548d1e04 100644
--- a/src/app/components/uia-stages/SSOStage.tsx
+++ b/src/app/components/uia-stages/SSOStage.tsx
@@ -56,7 +56,7 @@ export function SSOStage({
         <Box grow="Yes">
           <Text size="H4">SSO Login</Text>
         </Box>
-        <IconButton size="300" onClick={onCancel} radii="300">
+        <IconButton size="300" onClick={onCancel} radii="300" aria-label="Cancel">
           <Icon src={Icons.Cross} />
         </IconButton>
       </Header>
diff --git a/src/app/features/room/RoomTimeline.tsx b/src/app/features/room/RoomTimeline.tsx
index 436bbe35a..466f0caf4 100644
--- a/src/app/features/room/RoomTimeline.tsx
+++ b/src/app/features/room/RoomTimeline.tsx
@@ -546,6 +546,17 @@ export function RoomTimeline({ room, eventId, roomInputRef, editor }: RoomTimeli
   const timelineRef = React.useRef(timeline);
   timelineRef.current = timeline;
   const eventsLength = getTimelinesEventsCount(timeline.linkedTimelines);
+
+  // Perf-5: precompute base offsets once per linkedTimelines change instead of O(N×T) scan
+  const timelineSegments = useMemo<Array<[number, number, EventTimeline]>>(() => {
+    let base = 0;
+    return timeline.linkedTimelines.map((t) => {
+      const len = t.getEvents().length;
+      const seg: [number, number, EventTimeline] = [base, len, t];
+      base += len;
+      return seg;
+    });
+  }, [timeline.linkedTimelines]);
   const liveTimelineLinked =
     timeline.linkedTimelines[timeline.linkedTimelines.length - 1] === getLiveTimeline(room);
   const canPaginateBack =
@@ -1809,7 +1820,20 @@ export function RoomTimeline({ room, eventId, roomInputRef, editor }: RoomTimeli
   let newDivider = false;
   let dayDivider = false;
   const eventRenderer = (item: number) => {
-    const [eventTimeline, baseIndex] = getTimelineAndBaseIndex(timeline.linkedTimelines, item);
+    // Perf-5: O(T) → O(log T) via precomputed segments
+    let eventTimeline: EventTimeline | undefined;
+    let baseIndex = 0;
+    {
+      let lo = 0;
+      let hi = timelineSegments.length - 1;
+      while (lo <= hi) {
+        const mid = (lo + hi) >>> 1;
+        const [base, len] = timelineSegments[mid];
+        if (item < base) { hi = mid - 1; }
+        else if (item >= base + len) { lo = mid + 1; }
+        else { eventTimeline = timelineSegments[mid][2]; baseIndex = base; break; }
+      }
+    }
     if (!eventTimeline) return null;
     const timelineSet = eventTimeline?.getTimelineSet();
     const mEvent = getTimelineEvent(eventTimeline, getTimelineRelativeIndex(item, baseIndex));
diff --git a/src/app/features/settings/Settings.tsx b/src/app/features/settings/Settings.tsx
index 5e1a20f4a..46452aa34 100644
--- a/src/app/features/settings/Settings.tsx
+++ b/src/app/features/settings/Settings.tsx
@@ -141,7 +141,7 @@ export function Settings({ initialPage, requestClose }: SettingsProps) {
               </Box>
               <Box shrink="No">
                 {screenSize === ScreenSize.Mobile && (
-                  <IconButton onClick={requestClose} variant="Background">
+                  <IconButton onClick={requestClose} variant="Background" aria-label="Close settings">
                     <Icon src={Icons.Cross} />
                   </IconButton>
                 )}
diff --git a/src/app/features/settings/about/About.tsx b/src/app/features/settings/about/About.tsx
index b47acd179..96d21a147 100644
--- a/src/app/features/settings/about/About.tsx
+++ b/src/app/features/settings/about/About.tsx
@@ -25,7 +25,7 @@ export function About({ requestClose }: AboutProps) {
             </Text>
           </Box>
           <Box shrink="No">
-            <IconButton onClick={requestClose} variant="Surface">
+            <IconButton onClick={requestClose} variant="Surface" aria-label="Close">
               <Icon src={Icons.Cross} />
             </IconButton>
           </Box>
diff --git a/src/app/features/settings/account/Account.tsx b/src/app/features/settings/account/Account.tsx
index c4b56e475..bf3e60324 100644
--- a/src/app/features/settings/account/Account.tsx
+++ b/src/app/features/settings/account/Account.tsx
@@ -20,7 +20,7 @@ export function Account({ requestClose }: AccountProps) {
             </Text>
           </Box>
           <Box shrink="No">
-            <IconButton onClick={requestClose} variant="Surface">
+            <IconButton onClick={requestClose} variant="Surface" aria-label="Close">
               <Icon src={Icons.Cross} />
             </IconButton>
           </Box>
diff --git a/src/app/features/settings/account/IgnoredUserList.tsx b/src/app/features/settings/account/IgnoredUserList.tsx
index 98db9459a..6c7dccc38 100644
--- a/src/app/features/settings/account/IgnoredUserList.tsx
+++ b/src/app/features/settings/account/IgnoredUserList.tsx
@@ -72,6 +72,7 @@ function IgnoreUserInput({ userList }: { userList: string[] }) {
                 size="300"
                 radii="300"
                 variant="Secondary"
+                aria-label="Clear"
               >
                 <Icon src={Icons.Cross} size="100" />
               </IconButton>
diff --git a/src/app/features/settings/account/Profile.tsx b/src/app/features/settings/account/Profile.tsx
index e982a7992..895012754 100644
--- a/src/app/features/settings/account/Profile.tsx
+++ b/src/app/features/settings/account/Profile.tsx
@@ -185,7 +185,7 @@ function ProfileAvatar({ profile, userId }: ProfileProps) {
                 <Box grow="Yes">
                   <Text size="H4">Remove Avatar</Text>
                 </Box>
-                <IconButton size="300" onClick={() => setAlertRemove(false)} radii="300">
+                <IconButton size="300" onClick={() => setAlertRemove(false)} radii="300" aria-label="Cancel">
                   <Icon src={Icons.Cross} />
                 </IconButton>
               </Header>
@@ -278,6 +278,7 @@ function ProfileDisplayName({ profile, userId }: ProfileProps) {
                     size="300"
                     radii="300"
                     variant="Secondary"
+                    aria-label="Reset display name"
                   >
                     <Icon src={Icons.Cross} size="100" />
                   </IconButton>
diff --git a/src/app/features/settings/developer-tools/DevelopTools.tsx b/src/app/features/settings/developer-tools/DevelopTools.tsx
index a3f04567b..63037ac18 100644
--- a/src/app/features/settings/developer-tools/DevelopTools.tsx
+++ b/src/app/features/settings/developer-tools/DevelopTools.tsx
@@ -51,7 +51,7 @@ export function DeveloperTools({ requestClose }: DeveloperToolsProps) {
             </Text>
           </Box>
           <Box shrink="No">
-            <IconButton onClick={requestClose} variant="Surface">
+            <IconButton onClick={requestClose} variant="Surface" aria-label="Close">
               <Icon src={Icons.Cross} />
             </IconButton>
           </Box>
diff --git a/src/app/features/settings/devices/DeviceTile.tsx b/src/app/features/settings/devices/DeviceTile.tsx
index 71b684f56..97c284a06 100644
--- a/src/app/features/settings/devices/DeviceTile.tsx
+++ b/src/app/features/settings/devices/DeviceTile.tsx
@@ -292,6 +292,8 @@ export function DeviceTile({
             outlined={deleted}
             radii="300"
             onClick={() => setDetails(!details)}
+            aria-label={details ? 'Collapse device details' : 'Expand device details'}
+            aria-expanded={details}
           >
             <Icon size="50" src={details ? Icons.ChevronBottom : Icons.ChevronRight} />
           </IconButton>
diff --git a/src/app/features/settings/devices/Devices.tsx b/src/app/features/settings/devices/Devices.tsx
index c957ab318..22a46f396 100644
--- a/src/app/features/settings/devices/Devices.tsx
+++ b/src/app/features/settings/devices/Devices.tsx
@@ -74,7 +74,7 @@ export function Devices({ requestClose }: DevicesProps) {
             </Text>
           </Box>
           <Box shrink="No">
-            <IconButton onClick={requestClose} variant="Surface">
+            <IconButton onClick={requestClose} variant="Surface" aria-label="Close">
               <Icon src={Icons.Cross} />
             </IconButton>
           </Box>
diff --git a/src/app/features/settings/devices/Verification.tsx b/src/app/features/settings/devices/Verification.tsx
index 6c7eab17b..5223df2cc 100644
--- a/src/app/features/settings/devices/Verification.tsx
+++ b/src/app/features/settings/devices/Verification.tsx
@@ -293,6 +293,7 @@ export function DeviceVerificationOptions() {
         size="300"
         radii="300"
         onClick={handleMenu}
+        aria-label="Verification options"
       >
         <Icon size="100" src={Icons.VerticalDots} />
       </IconButton>
diff --git a/src/app/features/settings/emojis-stickers/EmojisStickers.tsx b/src/app/features/settings/emojis-stickers/EmojisStickers.tsx
index 93715120c..85762b4f6 100644
--- a/src/app/features/settings/emojis-stickers/EmojisStickers.tsx
+++ b/src/app/features/settings/emojis-stickers/EmojisStickers.tsx
@@ -30,7 +30,7 @@ export function EmojisStickers({ requestClose }: EmojisStickersProps) {
             </Text>
           </Box>
           <Box shrink="No">
-            <IconButton onClick={requestClose} variant="Surface">
+            <IconButton onClick={requestClose} variant="Surface" aria-label="Close">
               <Icon src={Icons.Cross} />
             </IconButton>
           </Box>
diff --git a/src/app/features/settings/emojis-stickers/GlobalPacks.tsx b/src/app/features/settings/emojis-stickers/GlobalPacks.tsx
index a9288728f..6c3edc1ad 100644
--- a/src/app/features/settings/emojis-stickers/GlobalPacks.tsx
+++ b/src/app/features/settings/emojis-stickers/GlobalPacks.tsx
@@ -373,6 +373,7 @@ export function GlobalPacks({ onViewPack }: GlobalPacksProps) {
                   variant="Critical"
                   onClick={() => handleUndoRemove(address)}
                   disabled={applyingChanges}
+                  aria-label="Undo remove pack"
                 >
                   <Icon src={Icons.Plus} size="100" />
                 </IconButton>
@@ -383,6 +384,7 @@ export function GlobalPacks({ onViewPack }: GlobalPacksProps) {
                   variant="Secondary"
                   onClick={() => handleRemove(address)}
                   disabled={applyingChanges}
+                  aria-label="Remove pack"
                 >
                   <Icon src={Icons.Cross} size="100" />
                 </IconButton>
diff --git a/src/app/features/settings/general/General.tsx b/src/app/features/settings/general/General.tsx
index 998aa90ef..45e357eac 100644
--- a/src/app/features/settings/general/General.tsx
+++ b/src/app/features/settings/general/General.tsx
@@ -1184,7 +1184,7 @@ export function General({ requestClose }: GeneralProps) {
             </Text>
           </Box>
           <Box shrink="No">
-            <IconButton onClick={requestClose} variant="Surface">
+            <IconButton onClick={requestClose} variant="Surface" aria-label="Close">
               <Icon src={Icons.Cross} />
             </IconButton>
           </Box>
diff --git a/src/app/features/settings/notifications/KeywordMessages.tsx b/src/app/features/settings/notifications/KeywordMessages.tsx
index 7f84d607f..5be38c399 100644
--- a/src/app/features/settings/notifications/KeywordMessages.tsx
+++ b/src/app/features/settings/notifications/KeywordMessages.tsx
@@ -80,6 +80,7 @@ function KeywordInput() {
                 size="300"
                 radii="300"
                 variant="Secondary"
+                aria-label="Clear keyword input"
               >
                 <Icon src={Icons.Cross} size="100" />
               </IconButton>
@@ -118,7 +119,7 @@ function KeywordCross({ pushRule }: PushRulesProps) {
 
   const removing = removeState.status === AsyncStatus.Loading;
   return (
-    <IconButton onClick={remove} size="300" radii="Pill" variant="Secondary" disabled={removing}>
+    <IconButton onClick={remove} size="300" radii="Pill" variant="Secondary" disabled={removing} aria-label="Remove keyword">
       {removing ? <Spinner size="100" /> : <Icon src={Icons.Cross} size="100" />}
     </IconButton>
   );
diff --git a/src/app/features/settings/notifications/Notifications.tsx b/src/app/features/settings/notifications/Notifications.tsx
index 095a9bba9..e7caae92e 100644
--- a/src/app/features/settings/notifications/Notifications.tsx
+++ b/src/app/features/settings/notifications/Notifications.tsx
@@ -23,7 +23,7 @@ export function Notifications({ requestClose }: NotificationsProps) {
             </Text>
           </Box>
           <Box shrink="No">
-            <IconButton onClick={requestClose} variant="Surface">
+            <IconButton onClick={requestClose} variant="Surface" aria-label="Close">
               <Icon src={Icons.Cross} />
             </IconButton>
           </Box>