It can take a long time to load user pages, scheduled reports, segment builder etc etc Speed the interface up for admin tasks.
This is very similar to http://ideas.omniture.com/t5/Adobe-Idea-Exchange-for-Omniture/SC15-Admin-Console-Speed-up-User-Manag...
However I will leave this one separate because it doesn't pertain exclusively to user management.
This note will remind us that a similar idea has a bunch of votes.
Yes indeed I had the job of creating 53 identical user segments apart from a single value and what I found that 1 out of about 3 segment builder loads took over 20 secs, this really impacted the length of time this task took.