[Epic] LR-4 · Performance & Sync Overhaul for Large Accounts

[Deniel9204]

Reduce sluggish room switching, app-wide animation stutter, and high memory on long-running sessions and large accounts; move toward sliding sync and streaming media.

Why it's long-running: Architectural: adopting sliding / simplified sliding sync (MSC4186/3575), memoizing sidebar/recent rebuilds, streaming attachment upload/download instead of whole-file in-memory handling, and profiling timeline/call hot paths per platform.

Tracked issues

• Sluggish room switching after long uptime #70 — Sluggish room switching after long uptime
• App-wide animation stutter on Android #71 — App-wide animation stutter on Android
• Slow media playback; downloaded media not instant #72 — Slow media playback; downloaded media not instant
• Removed members slow to update in member list #73 — Removed members slow to update in member list
• Cache profile information #74 — Cache profile information
• Adopt sliding sync for large accounts #75 — Adopt sliding sync for large accounts
• Unread indicator broken against Continuwuity #76 — Unread indicator broken against Continuwuity
• Memoize sidebar getSpaces and home-screen recent re-sort

Related (tracked under other epics)

• Room list / sidebar shows no rooms on Web & Linux #13 — Room list / sidebar shows no rooms on Web & Linux
• Background/queued upload with progress, retry, compression #35 — Background/queued upload with progress, retry, compression
• Network resilience: DNS-retry, WebGL-less freeze, ramdisk #68 — Network resilience: DNS-retry, WebGL-less freeze, ramdisk

_{Epic LR-4 · area: performance · auto-generated from ROADMAP.md §5.}