For a while now when I switch between two user sessions the inactive one in some cases will stop executing GUI processes. This way I'm forced waiting for another session to load and can't do something in another one, because if I switch and return after a while it's still on the same point. The software affected includes rsync, rdiff-backup, KDE background processes, Steam launched in Openbox session (even downloading stops) and at least some games. VLC is not affected, at least it skips to needed time after I switch back, but for a moment I can see old frame and time in it's window.
What process is responsible for such behaviour? Can I reconfigure it somehow?
Update: Memory and Swap Usage
Some additional context: The irony is I suggested this exact behaviour on the old brainstorm.ubuntu.com (now defunct).