Converting Registry to mod_perl handle
First, use Apache::Request instead of CGI.pm, second use Apache::Cookie instead of CGI::Cookie (or CGI.pm's cookie stuff). This alteration was all relativly easy, just change $cgi_query (CGI.pm object) to $r, setting a value in $r is different and also you have to do $r->print() instead of just print.
The cookie stuff took me a while to get, just remember each cookie is an object not a hash! and then all becomes clear.
Just hitting the search (so it shows the default form) is what I based my test on just to get a base feeling for the speed rather than complicate it with DB connections and the like. Here are the results:
Method Req/sec time per request
Standard CGI: 3.36 297.56 ms
Apache::Registry: 10.20 98.43 ms
Mod_perl: 73.31 13.64 ms
The jump between CGI and Registry I was expecting and as I am using Template Toolkit I was also expecting a similar (well, double to trebble) jump in performance from Registry to mod_perl. To get 7 times more requests and increase in speed was beyond anything I could have expected.
So, the lession, mod_perl good! Take the time, have a go it can make an amazing difference.