Dario, one thing you may wish to try is to load the machine with RAM, at least temporarily, when you are adding this many users. Then, create a RAMdisk using the RAMFS filesystem (not sure if included in the default Debian kernel) and put the Postgres data files there. Create your users, then stop the Postgres database and copy the files onto your hard drive.
This should greatly speed up insertions, since the limiting factor is the speed of the hard drive. Since you are just loading users at this point, the database should not be too large, perhaps under 1GB.
