Seems like the consensus is that things have improved drastically since the "quantum" update, although there have been a handful of issues on specific hardware (which may or may not have been fixed in the meantime).

I'd suggest to just try it, it's completely free and pretty straightforward to install after all.

Firefox got much faster on November last year (FF 56)

It feels faster on some sites, a bit slower (but not unbearably so) on others.