Photoshop perspective warp very slow (on Mac)

After selecting the layout in perspective warp, when clicking 'warp' in order to apply the distortion, sometimes Photoshop displays the busy spinning wheel for several minutes, before allowing the layout pins to be moved.  This doesn't happen all of the time.  Sometimes it is instant, sometimes a few seconds, but sometimes up to 5 minutes.  And the efficiency indicator is always at 100%, so it's not a scratch disk issue.

I have allocated more RAM, closed and opened the app, deleted history but nothing seems to make any difference.  And my iMac is brand new and certainly fast enough.  Like I say, sometimes it is instant.

Any ideas?

Paul Watmore