Segments are very slow! Any tips?

I'm a very unhappy user who's relied heavily on tags up until we went over 10k subscribers. Now I'm fighting with segments that are so extraordinarily slow to populate. 

I'm scheduling a series of 3 emails to go out a week apart from each other. One goes to List A today. The next goes to List B next week.  The third email goes to a larger list of 400, but not to those who are also members of List A or B.

This last one I've created a segment that is just perpetually populating for the last 20 minutes. I'm unable to complete the scheduling until it finishes!

I need to leave work, and I didn't realize it would take this long to get this done. Am I doing something wrong?