We should offer a manner of monitoring the performance of Mjolnir

I have the suspicion that `is_user_banned` and `is_room_banned` can be quite slow if too many users show up in the list. We should find a way to monitor this.

Start by locating the is_user_banned and is_room_banned functions and reviewing how they process large user lists. Determine what performance signals should be monitored and how success will be measured; the work is done when a practical monitoring approach is defined and implemented for these checks.

Written by the indexing model from the issue text.