I can't see why it would take different amount of time depending on the message itself, so I can't answer that. Try running from commandline and see what happens.
Now some nitpicking: Your From address is incorrect. It should contain a valid email address. It should at least look like:
From: Mail from website <website@domain>
As other monks have said so often before, use warning, use strict etc etc bla bla.
As long as you don't take $to from the user I don't think there is any risk for spamming anyone but yourself, so I disagree with Anonymous Monks previous node. A bit more errorchecking, and checking for backticks and stuff wouldn't hurt though.