Every RoomEvent.Receipt (debounced 150 ms) calls computePositions, which iterates room.getJoinedMembers() and calls room.getEventReadUpTo per member on top of an O(timeline) index build, then sets a new Map as state. That map is the ReadPositionsContext value, so every Message in the rendered window re-renders — regardless of whether its receipts changed. In a large, active room receipts arrive continuously, giving a full timeline re-render every 150 ms.
How to trigger
Open a room with a few hundred active members and watch React DevTools while others read.
Suggested fix
Diff against the previous map and reuse the old reference (and old per-event arrays) when unchanged, and/or move the per-message lookup to a useSyncExternalStore-style subscription keyed by event id so only affected rows re-render.
Filed from the September 2026 client audit (branch lotus @ 4bea4895).
**Severity:** medium · **Type:** performance · **Confidence:** high
**Location:** `src/app/hooks/useRoomReadPositions.ts:21-64`, `src/app/features/room/RoomTimeline.tsx:466,2131`
### Problem
Every `RoomEvent.Receipt` (debounced 150 ms) calls `computePositions`, which iterates `room.getJoinedMembers()` and calls `room.getEventReadUpTo` per member on top of an O(timeline) index build, then sets a **new `Map`** as state. That map is the `ReadPositionsContext` value, so every `Message` in the rendered window re-renders — regardless of whether its receipts changed. In a large, active room receipts arrive continuously, giving a full timeline re-render every 150 ms.
### How to trigger
Open a room with a few hundred active members and watch React DevTools while others read.
### Suggested fix
Diff against the previous map and reuse the old reference (and old per-event arrays) when unchanged, and/or move the per-message lookup to a `useSyncExternalStore`-style subscription keyed by event id so only affected rows re-render.
---
_Filed from the September 2026 client audit (branch `lotus` @ 4bea4895)._
jared
added this to the Audit 2026-09 · Medium & Low milestone 2026-09-12 01:51:08 -04:00
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Severity: medium · Type: performance · Confidence: high
Location:
src/app/hooks/useRoomReadPositions.ts:21-64,src/app/features/room/RoomTimeline.tsx:466,2131Problem
Every
RoomEvent.Receipt(debounced 150 ms) callscomputePositions, which iteratesroom.getJoinedMembers()and callsroom.getEventReadUpToper member on top of an O(timeline) index build, then sets a newMapas state. That map is theReadPositionsContextvalue, so everyMessagein the rendered window re-renders — regardless of whether its receipts changed. In a large, active room receipts arrive continuously, giving a full timeline re-render every 150 ms.How to trigger
Open a room with a few hundred active members and watch React DevTools while others read.
Suggested fix
Diff against the previous map and reuse the old reference (and old per-event arrays) when unchanged, and/or move the per-message lookup to a
useSyncExternalStore-style subscription keyed by event id so only affected rows re-render.Filed from the September 2026 client audit (branch
lotus@4bea4895).