The slowest user-felt path is typing into the composer while the
assistant is streaming. Profile (scripts/profile-under-stream.mjs):
FadeText measureOverflow self time: 35.8 ms → 18.1 ms (-50%)
total active CPU during 7s window: ~150 ms → ~50 ms
Two changes in src/components/ui/fade-text.tsx:
1. Drop the `useEffect([children])` that re-ran `measureOverflow`
(reads scrollWidth + clientWidth — forced layout) on every parent
re-render. `useResizeObserver` already fires the same callback on
mount and whenever the host span's box size changes; that covers
the only case where overflow state can legitimately change. The
previous explicit useEffect was a forced-layout flush on every
parent render, which during streaming meant every token tick.
2. Wrap the component in `memo` with a custom comparator that
short-circuits the entire render when scalar string `children` and
the className/fadeWidth/style props are unchanged. The hot path
was tool-fallback's title chips being re-rendered by parent
streaming updates even though their text was stable; memo+
comparator skips that.
Also adds two harness scripts under apps/desktop/scripts/:
- latency-under-stream.mjs (key→paint latency while a turn streams)
- profile-under-stream.mjs (CPU profile while a turn streams)
Updates profile-typing-lag.md with the streaming numbers and confirms
the Enter→paint submit path is already fast (≤320ms on the populated
session; the 2s "stall after Enter" the user noticed once was a
one-time cold-start, not reproducible at the UI layer).
I'd guess the felt jank in real use is fast-burst typing during a
long-form streaming reply (code blocks + markdown lists multiply the
per-token render cost). The CPU savings here scale linearly with
token volume.
