Perl isn't even remotely close to being "one of the fastest" programming languages, which isn't overly surprising given its architecture.

You're thinking of Doug Bagely's Shootout, in which he neglects to actually optimize PHP because he feels that the average person wouldn't. That's fine and dandy, but you won't find production environments like that, unless they're filled with people that will be taking that sales job at Radio Shack in the next month or two.

http://www.bagley.org/~doug/shootout

You'll see many comparisons for performance here. Note that this is a pretty small set of things that one will actually do when you program for a living, so you shouldn't really base your life decisions on it. His methodology isn't perfect, and many of the submitters (of which I am one) have done things they wouldn't do (for the sake of clarity) just to squeeze on that extra bit of time.

Perl is a very complex language, and it's impossible to learn in three days. You can learn a subset of Perl very quickly, but that's hardly synonymous with fluency. It's a novel and effective tool, but it's a rather inelegant language. This is something Perl 6 hopes to address.
The real reason it's used by a lot of sites, though, lies mostly in its age and use in history. People learn Perl, because other people have used Perl for web work, and then the cycle goes on. These people are hired out, and it becomes a defacto standard. There's no harm in that, but it really has nothing to do with Perl's technical qualifications.