importing users was >7000/sec for my case, and it increased gradually during the process.
I am using MacBook Pro, the SDD speed is big advantage.
Your forum is big and may used buggy MySQL versions in some occasions before, it may be also crashed, rebooted … several times. In addition, this bulk importation is so beta and need improvements. So, I think you should try modifying some code and running on local too.
P/s: I had to write some more code like custom BBCode parser, attachments, user avatars importing… to make my import perfect.