This test appears to run very fast on all other bots (including debug WK2), but it times out on debug WK1: https://webkit-test-results.appspot.com/dashboards/flakiness_dashboard.html#showAllRuns=true&tests=fast%2Fselectors%2Fnth-last-child-of-register-requirement.html On debug WK2, it takes just under 30 seconds.
Marked as slow in r180208.
It's weird, I wonder why the boundary tests are slow in debug.