Google PageSpeed is the more common of the two in our index: it runs on 92 websites against 27 for mod_perl. Counts come from real scans, not vendor claims.
Google PageSpeed is a family of tools designed to help websites performance optimisations.
Mod_perl is an optional module for the Apache HTTP server. It embeds a Perl interpreter into the Apache server. In addition to allowing Apache modules to be written in the Perl programming language, it allows the Apache web server to be dynamically configured by Perl programs.
Want to know which one a particular site uses? Look it up and see its whole stack, or compare two sites side by side.