The 20000-file measurement had a shape worth following up: median 184 frames, but one iteration of five at 598. Reading the listing path found the reason it could be that shape. AllFilesSource.list already hops to Dispatchers.IO, but BrowserViewModel.recompute filtered and sorted whatever it returned on the caller's thread - and the default order is natural order, which parses the digits in every name on every comparison, so 20000 rows is a few hundred thousand string comparisons plus a fresh 20000-element list inside a tap.
recompute now sorts off the main thread above 500 rows and keeps the synchronous path below it, so the common case is untouched and the tests that read state immediately after an action still hold. The dispatcher is injected for exactly that reason, and the large path has its own test: 600 files, listed, in natural order. The state update is guarded on path, sort and hidden-files so a second tap cannot be overwritten by the answer to the first.
The fix is not re-measured: the 20000-file fixture takes half an hour to build through MIUI's storage layer, so PERFORMANCE.md keeps the 598 outlier as the motivation and says plainly that whoever has the fixture next should confirm the maximum came down.
SoftwareApp
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