Just as a note, if you're doing heavily performance-intensive stuff, you can use the latest version of Cython to auto-cythonize (i.e., compile down to C) your code. This requires you have Cython > 0.11.2 or so installed.
At the top of your script using yt, put this line:
import pyximport; pyximport.install(pyimport=True)
And it'll try compiling to C all of your code before executing. Without annotations, speedups of 10-30% have been reported. Currently, even after hacking out the "exec" calls, it doesn't work with yt ... but I'm going to play with this some more later.