I'm guessing you've already happened across this, but in case you haven't, here is an explanation of the the compilation flags thorium uses for it's chromium builds:

https://thorium.rocks/optimizations

It seems as good of a place to start as any.

I haven't spent a ton of time down this particular rabbit hole, but now I may have to.

Immolo has a video on this subject https://yewtu.be/watch?v=umiVJdnZxMw

I realize what sub this is but really your best bet is probably to use a fork with optimized binaries (mercury comes to mind) it’s the best way to get fast firefox on arch too lol. the main repo package is fairly poor (on arch)