Faster than the current Mozilla builds of Tamarin, which have not been optimized and in many situations are still slower than SpiderMonkey.

On the other hand, it bears noting that Tamarin isn't going to make it into shipping code until Mozilla 2.0--presumably that will be Firefox 4, but it's still so far off that I believe that determination hasn't even been made yet. Whereas on past experience I would expect SquirrelFish to be in a shipping Safari build much sooner.