See the durations over time:
https://chromium-swarm.appspot.com/tasklist?c=name&c=state&c=created_ts&c=duration&c=pending_time&c=pool&c=bot&et=1518305340000&f=master%3Achromium.chromiumos&f=name%3Aunit_tests&f=buildername%3Alinux-chromeos-rel&l=1000&s=created_ts%3Adesc&st=1518132540000

Here's the first build with the slow-down:
https://ci.chromium.org/buildbot/chromium.chromiumos/linux-chromeos-rel/5097

Nothing very obvious in the blamelist. https://chromium-review.googlesource.com/884503 looks potentially the most culpable? (At least, to my untrained eyes it does.) So assigning to author. If the change is unrelated, feel free to unassign yourself.