Regular Expression Matching Can Be Simple And Fast, by Russ Cox:
Perl [and PHP and others] could not now remove backreference support, of course, but they could employ much faster algorithms when presented with regular expressions that don’t have backreferences.
How much faster? About a million times (no, I do not exaggerate).
I use a lot of regular expressions, and relatively few of them use backreferences. It’d be worth optimizing.