I have noticed that some people have noted that running PHP as a CGI program can run slowly compared with a compiled in module. Some have noted that they want to use FastCGI but are hesitant. I found that using the Apache 2's CGID module was a great way to speed up performance almost to the same level as an "so"-installed PHP module but you get the added benefit of running each virtual host under it's own user and group.
In my testing I got 44 pages per second using PHP as a module and I got roughly the same performance (within 5%) running PHP as a CGI program through CGID.
CGID is also really easy to set up. Just add --enable-cgid to your Apache configure command and you're good to go. Just set up PHP as a CGI normally.
I'm sure that there's extra RAM used for this method but RAM is as cheap as borscht anyways so it shouldn't be a major factor when trying to speed up PHP CGI.