I think you'll find it does pretty well if you implement something from scratch and follow and take advantage of standards. PHP/Java-using retards that use Prototype for checking a couple of form fields can drag it down to what is basically interpreter-in-an-interpreter performance, but I haven't had any problems iterating over and running regular expressions against a megabyte of data in tens of milliseconds, which is good enough for UI stuff.
I'm sure there's still room for improvement, but the best implementations are pretty decent for a securely sandboxed language of that dynamicity. See for instance
http://shootout.alioth.debian.org/u64/benchmark.php?test=all&lang=v8&lang2=java , where though none of the ECMAScript engines are really geared towards number crunching, results are mostly within order-of-magnitude.