Hi there,
A few months back, I complained about the slowness of the image function in matplotlib. One of the cullprit was a slow clip function; I've done a bit some work to improve the situation on numpy's side, efforts which were integrated in numpy 1.0.2. Now, when you clip a numpy array with scalar min and max values, you get a 5 to 30 fold speed-up; to get the maximum efficiency, you need inplace clipping (using the syntax a.clip(min, max, a) for a a numpy array). This makes image significantly faster (between 100 and 200 ms on recent computers), and I am sure in other functionalities of matplotlib as well.
cheers,
David