Have you looked at about:performance and also when you hit this, then create a perf profile using https://perf-html.io/ https://perf-html.io/ and submit it as a

Have you looked at about:performance and also when you hit this, then create a perf profile using https://perf-html.io/ https://perf-html.io/ and submit it as a bug to bugzilla.mozilla.org. If possible do both using Nightly. In my experience the developers are very(!) welcome to good bug reports.