I was a Firefox evangelist from version 3 all the way until 2014 or so. I moved on to Waterfox, then to Palemoon, then to Cyberfox. Problem is, the engine was getting old and slow. I kept holding on all the way until 2017 when I couldn't take it anymore. I now use Chromium and Comodo Dragon (Chromium-based) which are lightyears faster. I tried firefox again just recently and it actually was as fast as they said but now I have no desire to move back to Firefox, at least at this time. I'm now used to my extensions in Chromium.
Firefox's engine was re-written just last year (v57), pretty sure it was done a few other times as well. Not that the age of a program is really an issue.
The others are not light years faster, start with clean browsers they all run relatively the same, the differences are super small. The problem is no one uses an even playing field in the real world, people forget plugins/extensions, these can create HUGE drags on the browser. One single specific plugin on FF sends my launch time from less than half a second all the way up to 3.5 seconds. Chrome worked around this issue partly by trying to run in the background on system start, this way you blame the OS for your slow boot times, instead of Google for slow launch times. So if you compare times from an old FF install to a new Chromium install, of course Chromium runs faster, you hadn't butchered it with plugins yet.
A lot of people who switched from one to another, if they later come back after getting a new system are surprised how fast FF is when in reality they just reversed the process, Chrome is now loaded with plugins/extensions while FF is all fresh and clean. FF 57 did actually improve things as well, despite complaints from long time users about how plugins were handled, which again, was probably part of the problem to begin with.
That's only part of the story.
Chrome/Chromium did have a slight advantage for a while on page loads, but there's a really good reason why. Firefox was using a small amount of cache, meanwhile Chrome was using hundreds of megs of your drive. So yeah it loaded pages faster, because it was sucking up a bunch of your drive and not actually fetching pages. You could do the same with FF by changing simple setting, though the opposite can't be said for Chrome, Chromium and anything based on it since Google hates you messing with their product. Chrome also stores backups of every update, I've seen older Chrome installs that were sucking down 2.5 gigs or more of drive space between cache and old versions, I'm not even sure there is a way to disable that.
While Chromium doesn't keep a backup of updates or run in the background, it does still share and send an awful lot of data to Google. I counted 7 connections to Google while launching to a blank startup page. I can't say on Comodo since there's is no Mac or Linux version and I can't be bothered to setup a Vm and firewall to test it, but since it's based on Chromium, it's not unlikely that it also has a few connections to Google and/or Comodo.