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.
Not just that, but in an earlier release code was shipped to set the pref for users who already had the file, so that anyone who already had one of these files would not observe any behavior regressions.
I can't claim credit for it; I hadn't even realized it had happened until I dug a little bit into the history while reading this thread. All I knew was that my userChrome.css was still working fine... ;)
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.