It's web builders deliberately building their sites and webapps for chromium based browsers only, because it has over 80% market share. They only test on firefox rudimentally. The experience is subpar and people use chromium instead because of it, cementing chromium as the most used browser. Some site builders do this because they don't have the time to extensively test a browser with low market share, others, like Google, do it deliberately.

Depends on what sites you are using, google sites are slower for me, others are faster.

Also looks like chrome is better at looking faster somehow, probably starts to render page sooner.

And of course: wgich extensions do you have in firefox and how old is your profile. Try it out with new, clean profile and than you will feel it.