If you're talking about Google owned sites, there's circumstantial evidence that Google sets their sites up to do that intentionally in order to gimp competitors.

I'll take slower over ad-bloated every day.

That's weird, something is definitely wrong. Are they set up in a similar way? The first thing that comes to my mind is: Are you using the same DNS server on both? Differences in DNS response time should be more noticeable than rendering time on most hardware. And I think Firefox doesn't use the system DNS by default but I might be wrong. Do you mind checking? I'm curious now.

For me, switching from chrone to ff around 3 years it felt the opposite. Ff opens so much faster. Also scrolling is way smoother.