(In reply to Henrik Skupin (:whimboo) [⌚️UTC+2] from comment #0)
Florian, do we have a way to maybe request more startup logging? If yes, I would happily enable it for our CI Mn test jobs.
It would be really nice to be able to look at a startup profile to figure out what's happening in that case. How easy is it to reproduce? Is this something that happens all the time on a specific configuration? Something intermittent that we could make force to happen with a few retriggers on try?
If you can somehow put the MOZ_PROFILER_STARTUP environment variable, and find a way to dump a profile and get it uploaded, we'll get the information we need.
Is there any chance we could make this slowness happen for the browser/base/content/test/performance/ folder of browser chrome mochitests? If so, if you make browser_startup_mainthreadio.js or browser_startup_syncIPC.js fail on try (eg. by removing some whitelist entries), you'll get profiles automatically uploaded with the failing jobs. I found bug 1551632 using this method.