You might also have problems with Java heap size - you can increase it to let the dispatcher do more work with more RAM.
There's absolutely nothing wrong with standing up lots of dispatchers to take care of an initial load. You'll almost certainly find that the issue is with the comms -> ABAP rather than the database. Each user will take X number of seconds to write and there's not a huge amount you can do about it.
Essentially, you're doing a large number of users rather than normal operations. I'd assign more resources to the task as probably the best option.
Peter