Yeah, sounds like a non-reason, my user CSS was like 5 lines.

I'll have to run an extension that will be injecting JS/CSS into every page, to get the same effect. That will likely be slower. Not a speed improvement exactly.

At least it's more powerful. I can replace any rule inside already loaded CSS. Useful to escape this braindead age of "font-size < 16px && font-weight < 400".

It has to have been showing up in profiles for them to bother, nobody would go out of their way to disable it otherwise. They added a pref to turn it back on and then disabled it instead of the easy thing (just removing it) which is some engineering effort plus testing effort to make sure the pref works right. Silly to do that for nothing.

Antivirus software does add a measurable delay to file operations sometimes, and each file is gonna be in its own sector so I could see them losing at least a few milliseconds there. Applying CSS does add overhead but the average user can't have that many rules in there so I suspect it's purely on the file i/o level.