Well, it's a 700k interpreter and runtime. It isn't slow for simple things, but only because JS is really fast now. Interpreters are usually 5 to 10 times slower than whatever they are running in.
But if you include a giant runtime, the compilation and evaluation of it on every page load, and downloading it (there's evidence to show many hit CDNs with cold caches, probably screwed up firewalls or proxies that mess with caching) can make for a suboptimal experience that can't be optimized without rewriting in something that doesn't require a bytecode VM embedded in your JS.
http://hashcollision.org/whalesong/examples/raphael-demo/rap... isn't bad
http://hashcollision.org/whalesong/examples/boid/boid.html is a little slow compared to http://graphics.cs.wisc.edu/Courses/Games12/Tutorial1/Phase1...
Some of the low framerate could be frame timing (whalesong could be using setTimeout poorly), but they seem to use about the same CPU.