Zaddion, are you using some kind of addon that auto promotes players out of the standard 'user' group?
ULX (or more accurately ULib) wasn't designed to store authentication details for every player who joins your server. As an Admin mod, the current system is really only meant to store data for administrators and maybe VIP or donator groups. When you start trying to Auth every player it can really inflate the file and when garrysmod goes to open and seek information from that flat file, it will cause your server to lock while it's doing that.
The only fix, really, is to trade up the standard flat file system for an SQL based system.
If you search for it, you'll find a couple snips of code that people have released (myself included) that will help you convert your ULX installation to use SQL.
This will require some basic knowledge and access to the root of your gmod installation (to install the SQL module which is a dll file) as well as access to a MySQL database (preferably on the same physical system as your game server).