If you’d like more info on why SquirrelFish is fast, head over to the Surfin’ Safari blog where developer Geoffrey Garen explains the details:
SquirrelFish’s bytecode engine elegantly eliminates almost all of the overhead of a tree-walking interpreter. First, a bytecode stream exactly describes the operations needed to execute a program. Compiling to bytecode implicitly strips away irrelevant grammatical structure. Second, a bytecode dispatch is a single direct memory read, followed by a single indirect branch. Therefore, executing a bytecode instruction is much faster than visiting a syntax tree node. Third, with the syntax tree gone, the interpreter no longer needs to propagate execution state between syntax tree nodes.
Safari nightly builds can be downloaded from the WebKit site.
Or you could just wait for the SquirrelFish engine to hit Safari and spend the interim pondering the super cool logo of a somewhat drunken-looking SquirrelFish.