Though it is not going to help you, the performance degrade is what is to be expected. We improved the adding of users into the OpenACS system lately (if I'm not mistaken), but due to the many issues with .LRN dependencies you are most likely going to wait for a couple of days before you finish with your 80000 users.
When upgrading from ACES to .LRN 1.0 it took us two days to import 80.000 users, but Tilmann is an expert in this as he wrote the import scripts.
All in all, install developer support and trace which queries take up the most time. If you are not sure what to do about them, post them here, so we might take a stab at this in the long run (as you will not be the only organization trying to mass import users).
