I've noticed a major slowdown of the macosx backend between version 1.1.1 and 1.2.0, when using pylab.show(). The following code times the call and gives very different results between the two versions:
dt ~ 454 pts/s for 1.2
dt ~ 168,480 pts/s for 1.1.1
A difference by a factor ~ 200. Can others test and confirm? The results did not change much when using TkAgg.

import matplotlib
#matplotlib.use("TkAgg")
matplotlib.use("MacOSX")
import pylab
import numpy
import time

N=10000

fig, axarr = pylab.subplots(1,1)
axarr.plot(numpy.random.normal(size=N))

matplotlib.interactive(True)

t0 = time.clock()
pylab.show()
t1 = time.clock()
ptPerSecond = 1.0*N/(t1-t0)

print("dt=%.3f points/s"%ptPerSecond)