In my experience speed differences between browsers are indeed negligible
on properly engineered sites/apps. Every so often I run into one that was only ever developed for and tested against Chromium and has major performance issues under Gecko and/or WebKit. Some of it might come down to poorly pruned or outdated polyfills, which in many cases are filling in for features that those engines gained support for quite some time ago.
I wish there were something that could thwart the “Chromium by default” assumption at large amongst devs, because it’s making the web worse than it needs to be.