A cold launch took 7.6 seconds: sources, dictionary, projection, then first paint. The fix was meant to be a content snapshot written on first launch and read on the next, which should have cut that to about a second and a half.
It didn’t. Every launch reported a cache miss, and for a whole release nobody noticed, because the snapshot code caught and discarded every error. An absent cache and an unreadable one looked identical.
The root cause took a while to find. A stale sideloaded package produced a
rejected read on every launch, and the snapshot logic declined to write
whenever the read recorded any failure. So no cache was ever written, silently.
Two fixes. The failure now travels in the snapshot payload, so a hydrating
launch can say what it’s missing. And only a failure that might still resolve
blocks the write: unreadable blocks, rejected and corrupt don’t.
With the faulty package removed, the next launch hit the cache: 7.6 seconds down to about 1.1, with the read at 299 ms and the decode at 323 ms. The lesson isn’t about caching; it’s that a cache that can fail silently is worse than no cache at all.