Say what a two-simulator run actually costs

Roughly one second of test time per minute of wall clock, measured with the host
healthy — 38 runnable processes, load 27. The cost is XCUITest snapshotting the
whole accessibility tree per query against a home screen carrying a stories
strip, a buddy list, a groups section and a tab bar.

A DM run is 20-40 minutes per side and the group phase has never completed in
one sitting. Worth knowing before someone schedules this as a quick check.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

PARITY.md

@@ -448,3 +448,16 @@ To confirm, instrument both clients' ratchet ids across a concurrent send. Do
 not patch on this reasoning alone: the file's history includes an
 advance-unconditionally bug that lost messages silently, and a careless fix here
 trades one silent loss for another.
+
+### Why confirming it is expensive
+
+⚠️ **A two-simulator run advances roughly one second of test time per minute of
+wall clock**, and this is not machine load — measured with the host healthy
+(38 runnable processes, load 27). The cost is `Capturing element debug
+description`: XCUITest snapshots the entire accessibility tree on every query,
+and this app's home screen carries a stories strip, a buddy list, a groups
+section and a tab bar.
+
+A DM run takes 20-40 minutes per side. The group phase has never completed
+inside a sitting. **Budget hours, not minutes** — or give the pair a trimmed
+screen to work against before running it.