The thread prefixes step isn't paginated, but it normally shouldn't take that long as the data is not incredibly complicated.
@Mike can I suggest you revisit pagination of thread prefix imports (if possible) for a future version?
I'm currently (as in right now) attempting to migrate my ZooChat forums from vB to XenForo and we have over 1,800 thread prefixes (one per zoo) across 116 prefix groups (one per country) ... as mentioned in this thread:
https://xenforo.com/community/threads/can-i-have-thousands-of-forums.120543/#post-1095900
I had a couple of hiccups with my test import, but got it working, but now my production import (on an identical machine to that I did the test on) is falling over with gateway timeouts and "Step threadPrefixes cannot be run" errors.
I'll try restoring from backup and increasing the timeouts to see if I can get it past this step.
Thread prefixes are critical to us, since we are using them to group threads by zoo - we have one forum per country (grouped into region categories) and each forum has one thread prefix per zoo for that country so users can either post a general thread about zoos in that country or post a thread about a specific zoo by attaching a thread prefix.
I do acknowledge that it's a rare use case though
Will report back how I get on. I was surprised at how well the test migration went and how well it performs with so many forums and galleries (we have over 2,200 photo gallery categories), but then that test server didn't have any load on it!