Hi. I've been browsing different threads and it seems like the general idea is that if I use PHP 5.3 CGI instead of FastCGI I will have lower RAM usage and a faster server. Is this true? Should I switch over all of my domains on the VPS to just PHP 5.3 CGI?
What I did was use a stop watch and 10 repeated loads of my homepage. Try with with fast CGI and without. On my sites fastCGI was slower.
Well that is one way of doing it.
Is there any reasoning why CGI would be faster than FastCGI?
No, the opposite should be true.
CGI has to start a new process for each and every request, where as FCGI is persistent. To say FCGI is slower is a bit like saying memory caching will make your site slower.
If you're using Wordpress, WP-Super-Cache + PHP CGI works best, giving you very low RAM usage. That's because you don't need PHP to serve cached files, saving you RAM.
You can read the details here: http://wiki.dreamhost.com/WordPress_and_PHP
That entire article needs to be reviewed.