The trouble is, performance isn't as simple as "X is faster than Y". It's a series of complex trade-offs. For example, mod_perl causes Apache processes to have a larger memory footprint. Depending on what you're doing, it may well be worse if you're running under Apache::Registry, especially if your CGI scripts aren't tuned to run under it. However, the execution time is generally far superior to CGI. In a lot of cases, it's better to use more memory than to run slowly.

Of course, this is not always the case. It's all highly dependant on exactly what you're doing, what the goals of the project are, etc.

There's a strong possibility that performance isn't as important for your application as you think. There isn't a huge amount of difference between the performance of mod_perl using custom handlers, mod_perl using Apache::Registry and PHP using mod_php, at least for the vast majority of applications. Unless your requirements specifically state 'X must run in n seconds or less', I wouldn't let performance be a huge factor in whether I chose PHP over Perl