i.e. about 10 categories in all, with between them perhaps 100 threads. It was an hangover from the VB import of a area of the forum we hid a long time ago and its time they were pruned off.
Anyway, the procedure timed out. (URL: /admin.php?categories/delete)
Fatal error: Maximum execution time of 30 seconds exceeded in <removed>\library\Zend\Db\Statement\Mysqli.php on line 70
Easy to get around (just delete all the forums one by one), but it still shouldn't timeout.