As said in sibling: Open a bug report at https://bugzilla.mozilla.org/

Mention Quantum Flow and "performance" in it. Make sure to attach a performance profile with https://perf-html.io/ using the linked add-on there.

I had many cases where the mozilla devs were glad about such bug reporst (especially if you are on Linux - I am too :) - which is not so often used and reported like Windows or Mac)