Single cpu speed is the important factor.
On my machines, a rate of 800-1000 users or posts/minute is fairly typical.
Note that when you do the final import, it’ll import only the users and posts that haven’t been imported already, so it won’t take very long.
Turn off the Normalize emails site setting (off was the default until recently). It should probably get turned off in this function here:
You can put it in your customized version of the xenforo script with SiteSetting.normalize_emails=false. I’m not sure what happened to those duplicate email users; there are two obvious things to do, give them a bogus email address or skip importing them. Looks like it gives them bogus ones? (And there’s a pretty good chance that they are, in fact, bogus users anyway). If it skipped them, then running the script again will import them.