The method I'm using is pretty inefficient I think...
I have a CSV with usernames and tags. It loops through, checks to see if the user already exists, if not, it inserts. It when loops through the tags and inserts those.
I couldn't really think of any other way to do it, as everything is associated to the voterID rather then the username.
Like I mentioned, I didn't really notice an issue with it until I got to ~1mil, and it sped up significantly when I removed ~250k registrants (only to slow back down).