"slightly faster" is a bit understated, you're usually talking about statically typed languages being an order of a magnitude faster for things that are computationally expensive. Granted most things aren't, so for a web page or whatnot, it probably doesn't matter; and certainly the expressive ability you gain from dynamic languages might be worth the trade off. But the trade off is undeniably there.
(Try comparing various languages here: http://shootout.alioth.debian.org/u32/benchmark.php?test=all... if you don't believe me)
Some dynamically typed languages can approach the speed of static languages, like Lisp, but they generally do that by introducing voluntary static typing hints, or having a JIT compiler introduce speculative code paths that guess the types coming in after some analysis.