diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 8eb28c886..4358ff5a8 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -216,9 +216,29 @@ jobs:
 
       - name: Run nextest tests
         env:
-          # Three concurrent workspace test links saturate the self-hosted
-          # runner's overlay I/O and can wedge Cargo until the 75m timeout.
-          CARGO_BUILD_JOBS: "2"
+          # #5394 mitigation, now under a measured experiment (backlog#1601).
+          #
+          # 2 was chosen when three concurrent workspace test links were believed
+          # to saturate the runner's overlay I/O and wedge Cargo until the 75m
+          # timeout. cgroup v2 readings from the sampler show the pod actually
+          # has 14 CPUs and 28GB (peak use 2.1GB), so 2 throttles compilation to
+          # a seventh of what is available and memory was never the constraint —
+          # the label name "sm-standard-4" had led everyone, including the
+          # original mitigation, to assume 4 cores.
+          #
+          # Raised to 3 on main pushes only; PRs keep 2 so the merge path is
+          # untouched while the experiment runs. Baseline over 17 samples at 2:
+          # median nextest/clippy step ratio 1.95, spread 1.85-2.06. The gate-2
+          # criterion is that ratio dropping at least 10% (below ~1.76) with no
+          # 75m timeout and no run showing three consecutive samples of
+          # rustc/collect2/rust-lld in D state. If it does not, the conclusion is
+          # "this limit is not the bottleneck" — fix it back at 2 and record the
+          # experiment, which is a result, not a failure.
+          #
+          # Must stay step-level: rust-cache hashes CARGO/CC/CFLAGS/CXX/CMAKE/RUST
+          # prefixed variables from process.env into the cache key, so promoting
+          # this to job level would rotate every key on this lane.
+          CARGO_BUILD_JOBS: ${{ github.event_name == 'push' && '3' || '2' }}
         run: |
           mkdir -p artifacts/test-and-lint
           ./scripts/ci/resource_sampler.sh start nextest