No worries, this issue has been haunting us for a while.
ULib was never intended to be used the way server owners use it, which is why we figured that a flat text file would work fine. If you only use ULX to store admins/moderators and some special groups it works great!
Server owners these days use auto promotion mods to assign groups to literally everyone that joins. Or gamemods like RP do that too. This will add up.
The new ULX4 will utilize SQLite I believe which will be a much more efficient way to store and retrieve player info for way more players.