diff --git a/docs/scheduler/ci.md b/docs/scheduler/ci.md
index af9df2c5..b5883a9a 100644
--- a/docs/scheduler/ci.md
+++ b/docs/scheduler/ci.md
@@ -234,6 +234,13 @@ following to your host config:
 }
 ```
 
+**Measure a collection on your own hosts before relying on that stagger.**
+`randomizedDelaySec` spreads the _start_, not the run, and the run is dominated
+by the sweep over unused `.links` entries — which scales with the size of the
+store, not with what this collection deletes. On a large store it can take most
+of an hour, so two hosts both set to `daily` may overlap however much jitter you
+give them. `journalctl -u nix-gc.service` reports each run's wall clock.
+
 **Remote builders:** if CI dispatches builds to a remote builder (for example via
 `nix.buildMachines` / `ssh-ng://`), the build outputs land in _that host's_
 store, so the same GC config should be applied wherever the builder runs —