That is not strictly true, the 'Regeneration Complete' dialogue box comes after a couple of seconds, but Pro/E remains extremely busy for about 5 minutes, before the 'Regeneration Complete' box disappears.
Task Manager reports CPU usage of over 50% i.e. one of the two cores is basically completely committed. It also reports that it is, indeed, Pro/E that is using all that processing.
I have looked at the trail file and it gets to '!ProgressBarDlg: Regen Completed' and shows nothing further.
How can I find out which feature is causing this problem? Is it just a case of moving the 'insert here' thing up and regenerating until the problem goes away, or is there a slick way?