The problem is that dotlrn::user_add triggers a huge number of other operations - depending on the number of other dotlrn packages installed, which are difficult to unwind. This is even more true when adding users to communities I believe, because then lots of personal data will be created for them for each dotlrn package installed.
Anyway, Dirks advice to install acs-developer-support is an excellent one. You might find one or two queries that account for the main part of the time the operation takes and find a way to optimize or postpone them.
