As someone who kept bouncing between Firefox and Chromium for quite a long time before finally settling with Firefox, I think that's the case only for light workloads with not a lot of tabs open. I was switching from Firefox to Chromium because Firefox was getting slow, but after a while of my normal usage, Chromium wasn't just getting slow - it was getting unusable, so then I switched back to Firefox (and the same cycle repeated after a while). But yeah, right after refreshing the browser profile, fresh Chromium felt a little bit faster than fresh Firefox. I don't think that's the case anymore.