I have used Request Tracker in the past. At a previous employer there were two installs of RT. We noticed that RT, at that time, did not scale well. As the database became larger it would become unresponsive. We were never able to determine the exact cause but I think there are several. For example, RT would not remove old session IDs from its table. This caused the database to have query tens of thousands of session IDs whenever a page was loaded. I created a script that would remove old sessions but performance was only marginally better. I can only hope that newer RT code has been improved.