Earlier quoted context omitted.
My experience is mostly on linux. Chrome was way better on all aspects i mentioned. It was like day and night.
Seems to me you had a corrupted profile or you installed the wrond extension(s). Have you tried comparing both on a brand new user profile ? A day and night difference on a fresh profile is unheard of, they should be comparable speed.
Create a fresh clean profile for both FF56 (or earlier) and chrome without any extensions.
Then, create/import a bookmark folder with 20 sites and sync them to both browsers.
Fire up each browser and watch the cpu usage with top (or task manager). Choose "open all in tabs" and monitor the cpu usage over time.
For me chromium finishes loading 3-5x faster. By "finishes loading" I mean all spinning circles in tabs are gone and the cpu usage drops to nearly 0%.
Firefox uses only 2 processes/threads, while chromium makes use of all cores. I am guessing the more cores your CPU has, the more obvious the difference. But you don't need an 8-core cpu to see the difference, the difference is as noticeable for me on my i5-6200U laptop.
But page loading speed and JS performance aside, what turned me away from firefox is the sluggish UI. Little things like how fast the dev tools open, or the speed at which right-click menus open. It's like firefox freezes for a millisecond on these operations while chrome feels much more fluid. I don't have a test case for this :)
I never really used Firefox, and I give it a go every year or so. I used Opera 12 for as long as I could precisely because of the FF UI responsiveness. Then Chrome came along. I keep looking back but... FF is not there yet for me.
I'm no fan of Google and I really hope the new FF is as good as people claim to be. Remains to be seen, having high hopes!
EDIT: Obviously if you have a slow internet link then the pipe becomes the bottleneck and both browsers will load sites at same speed. But in that case you could just close all tabs and reopen them and let the browsers load most of the stuff from cache.