Make sure you "vacuum --full --analyze yourdb" before you start the migration of users. If you haven't done that on your db, especially after installing a lot of packages and inserting many rows, the DB becomes considerably slower than just after being vacuumed. If it is possible to pause the migration and vacuum every 500 users or so that may help speed the process up somewhat ...
Neither would it surprize me if you'd get a big boost in the migration performance if you were able to locate and temporarily drop some relevant indices just to recreate them when the process is over, of course. (An index always causes a slow-down in insertion performance on a table althogh it in most cases drastically decreases the time it takes to select rows from it.)
The question of course is which index to drop and which to leave all alone 😊
Just my $.02
/Ola
