but Mod_Security is basically just for security, nothing more, nothing less.
It allows you to setup rules which prevent some common scripts not to be exploited.
Mod_Security really doesn't help with performance, it can actually slow things down a bit if there is to many rules or if the server is not powerful and gets to many hits.
Performance from apache can be improved by removing unneeded modules and optimizing certain settings in the httpd.conf. I don't know of any mods off-hand that increase performance, but there is a few other ones for security.