To zombie this thread and help anyone else experiencing this issue I recommend the following.
I used this method over a week ago and have quite a few users and very large libraries.
Back up your DB first, and YMMV.
Concurrent streams are up quite a bit after this change for me, so users seem to like this change as well. 
https://github.com/Cloudbox/Community/wiki/Speed-up-Plex
1. Stop Plex
2. Locate your Plex DB. cd plex/Library/Application Support/Plex Media Server/Plug-in Support/Databases
3. sqlite3 com.plexapp.plugins.library.db
PRAGMA default_cache_size = 6000000;
Press CTRL + D
4. Start Plex.
