If this is a per-site issue then maybe just use the devtools to profile what's h...

If this is a per-site issue then maybe just use the devtools to profile what's happening.

If it's a whole-browser issue then use the firefox profiler and configure it to profile from startup: https://profiler.firefox.com/docs/#/./guide-startup-shutdown

Maybe something obvious pops out.

It's not a per-site issue. It doesn't reproduce on other browsers such as Epiphany, Chromium/Chrome, etc.