One possible solution - put your userDB in a real DB (they are your crown jewels, and deserve a bit of special care), cache user data in a fast cache (local Mongo, Redis over a memory socket, Memcached, whatever - you do have a cache, right?), and let cache invalidation update comment names in due course.

I'm pretending there's two types of data - the "nexis" of the star (comments), which needs a fast DB, and the leaves (users), which are small enough to be cached.