diff --git a/docs/PERFORMANCE.md b/docs/PERFORMANCE.md
index 702b3d5..935d21c 100644
--- a/docs/PERFORMANCE.md
+++ b/docs/PERFORMANCE.md
@@ -123,6 +123,34 @@ lasted. It says the interaction is short (14 frames is about a quarter of a seco
 say whether any of those frames was slow. A device that reports frame durations — an emulator or any
 non-MIUI phone — is what turns this into a jank number.
 
+### How it scales with folder size
+
+The same test, run against folders of different sizes on the phone — only the fixture changed
+(`./tools/benchmark-fixture.sh --count N`):
+
+| files in the folder | frames to open it (`openAFolderOfAThousandFiles`) |
+|---|---|
+| 1 000 | **12–14** (two runs) |
+| 5 000 | **41** |
+| 20 000 | **184** (min 166, **max 598**) |
+
+The shape is roughly linear, which is what a `RecyclerView` fed a pre-sorted list should do: five times the
+files cost 3.5 times the frames, four times the files cost 4.5 times. A superlinear jump here would mean the
+listing had started doing per-row work that belongs to the folder as a whole.
+
+The **max of 598 at 20 000** is the number to keep an eye on. The median says the folder opens in about
+three seconds' worth of frames; one iteration of five took three times that, so at that size the first open
+is not merely slow but occasionally stalls — worth watching rather than explaining away, and the reason the
+20 000 point is the one a release should be checked against.
+
+Building that fixture is the expensive part, not measuring it: 20 000 files took about thirty minutes
+through MIUI's storage layer (`: >` in a loop), against roughly four minutes of benchmark on top. Worth
+knowing before anyone re-creates it, and an argument for the fixture being built once and kept.
+
+The emulator supplied the jank *metric* this phone cannot, at the 1 000-file point: `frameDurationCpuMs`
+P50 218 ms, P90 330 ms, and `frameOverrunMs` P50 315 ms. Software-rendered, so the values are not
+comparable with the phone's — what they prove is that the metric exists where frame timelines exist.
+
 One bug had to be fixed before `search` could be measured, and it is worth knowing about because it looked
 like a UI problem and was a state problem: the search iteration ends with search mode open, and with
 `Ignore()` nothing reinstalls the app between iterations, so the *next* iteration found a search field