Why is the snapping of frames so slow?
It seems that preview can be used with a callback to do real-time processing of images. However, snapping a frame, then calculating and displaying the result is very slow.
What is the best way to approach (quasi) real-time video processing?
I think that this example might help with your issue: http://www.mathworks.com/help/imaq/examples/acquiring-a-single-image-in-a-loop.html.