Interesting suggestion received via PM. I like it:
Maybe, for the FREE folks, have a limited list of say 5 filters. For the premium, unlimited. That might be your ticket. Seems the volume is up there. Allow filtering, but just allow a limited number for everybody. Simliar to posts. Upgrade and you get unlimited.
Though heavy use of filtering (it has been more prevalent among free members than subscribers) does impose a lot of overhead, and still would impose overhead with small filter lists for free members, the hit would be much smaller.
I'm thinking a much smaller list, like 2 or 3, for free members.
Thoughts anyone?