First I should note that I have no actual benchmarks to back any of this up

That said, I don't think you're going to find an appreciable performance difference between using C, php, or perl for your CGI programming. Usually, when any of these do not perform with great efficiency, it's because it's not properly optimized, not because it's not the "right" choice. I like to use perl CGI for all my web programming, but that's because I'm also responsible for writing the back ends that my pages will interface with, and using perl for all of it means I only have to

use CGI::Pretty;
use MyBackEndInterfaceModule qw( :client );
[download]

and off I go, never having to write the interface into the front end; the backend provides a simple API, including functions or methods to interface with it. This also means I can change from a pipe based interface with the backend, to sockets, to shared memory without having to make any changes in the front end. Which will be appreciated if somebody else is writing the user interface, and you haven't done benchmarks to decide which way is fastest yet. For me, it's the ease of implementing this sort of flexibility that makes perl my first choice.

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