As I have mentioned, my experience is about both: a 10 year old version of FF was crazy slow, Quantum is tolerable but still slower than Chrome, gets even much slower as soon as you install some extensions and still is uncomfortable to use on old PCs (e.g. 2013 MacBook Air). Chrome works perfectly fast on everything incl a >10yr old Core 2 Duo laptop.

I am a huge Firefox fan and don't even mind some tolerable amount struggle but have to admit this.

I once tried to migrate my girlfriend (who has the 2013 MacBook Air) to Firefox (Quantum). As she tried it she immediately told me she doesn't want it because it feels slow. Chrome apparently has some bug which makes eat up 100% of the CPU on the same machine often but works faster even when it does this.

This still has to be addressed if we want Firefox to become more popular. "Just disable all the extensions" (mostly meaning featured extensions recommended by Mozilla, e.g. uBlock Origin) is not a good answer, neither is "buy a powerful modern computer".