PS 6CC slow when working with layer mask

When working with a layer mask in PS 6 CC, there is a great deal of lag between what is brushed

out and the movement of the brush.  When the red is turned on (\), it becomes viewable in sections

very, very slowly.

Then, everything stops and task manager shows PS6 is "not responding".

Other functions in CS6 don't seem to be slow, and the program does not go to "not responding"

with any other tool.

Before using the layer mask, I crop the file to 1800 x 1200 @ 300, so I'm not working with

huge files.

This problem is recent.  Layer masks have never been a problem before.