Follow up for MDL-30668, will be good to remove the user preferences for the courses that are not used any more. All preferences are stored in the session and session may grow too big over time.

This could be observing deleting course and/or unenrolling teacher. Or there could be a cron task that deletes settings for the courses that have not been accessed for a while.
Or there could be some regular clean up on user log in.