I think I can relate to that issue and posted a similar topic some time ago: Very slow server respond times when creating thumbnails it seems they could be related to each other.
But the problem still exists that it lasts sometimes multiple seconds to get the thumb after a resize or crop. Any idea?
Got the same very very slow first respond times (~10 seconds) on localhost when I have a page with lots of images where thumbnails get generated.
Do you have the same behavior like this?