I took a look into the lag issues using your users.txt file, and it turns out that I had already implemented most of the optimizations I was talking about, and I wasn't really getting any major horrible lag with it
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EDIT: Oh wow, there we go. Yah, that's different lag than I thought it was lol. It looks like it's only triggered when you have the "regular" group (or which ever group has a crap ton of users) selected in the groups tab when someone joins, so you can test it on your server to see if it still happens. I'll start looking into a fix
EDIT2: Wow, that really is horrible, nearly crashed my gmod when forcing an update of the whole users table lol
EDIT3: Okay, I made a bunch of optimizations on this, will be pushing to SVN in a few minutes. It should help out anyone who has this problem in the future!