not build in.
SME Server is not designed for so many users.
It may work with such an amount, but I doubt, that the developers will change the behaviour of each and every panel, that has to cope with so many users (but if you donate, maybe they will do so!?).
But since the users (and other objects like ibays and groups) are still managed in an ASCII file, the access speed and sort and search features cannot be programmed efficiently.
Just curious: How long does it take? How fast is the server, how much RAM?