More efficient group cache reloading

Currently it looks like whenever the group cache is expired, it gets fully refilled when a user logs in. This might need quite some time. In one case I have seen a user waiting for more than 30 seconds before the server comes back to acknowledge the login.

It seems the code loads every group and user separately to check for membership attributes. Maybe it is simpler at least to load all affected users in one query.
However I am not familiar with all use cases, so this is just some idea which might be impractical. In that case please close as "Wontfix".