Anecdotally.. Firefox was always slower rendering pages compared to other browsers. On some sites that I visited regularly, using FF was depressing. Later on, it started to suffer from caching issues on my machine. This caused a massive system slowdown.
Tried Palemoon (non-blink\webkit, etc). It blazed through pages- even the heavy ones FF had problems with historically. The rendered results were spot on too. It was like getting a new machine.
What I'm saying is that we can't blame everything on pages being optimized for Browser X or Browser Y. Sometimes we need to call out our favorite browsing application and let them know this is an issue. Otherwise, they keep losing users based solely on performance.
Having said that, looking forward to Quantum.