In the late 00s I remember turning on an old computer with a 650 MHz Athlon CPU and being surprised that web browsing performance in Firefox wasn't bad. Now if I try that with a 1 GHz Pentium 3, performance is absolutely horrible. Is this why?
Basically -- ultimately, it's the rise of javascript for third-party content (ads, trackers, APIs, add-ons, etc etc) that drives both page bloat and poor rendering performance.
This, and Javascript, and browser bloat.
Unless the browser ends up swapping a lot I doubt that browser bloat would cause significant perceptible performance regressions compared to the quite significant amount of optimization that has happened in the meanwhile.