The Gnome menu -> Log out (username) -> Log Out command seems to be broken on my test laptop. It logs out after 90 seconds or more, but sometimes much less. Gnome -> Shut Down or even Gnome -> Log out -> Switch User works as normal.
If I leave it to log out and switch to a console, I can see two processes are still running under my username. One is qjackctl-wrapper, which I guess isn't being properly killed. The other is sleep - I don't know why that's still running.
I found I could force an immediate log out by clicking Gnome -> Accessories -> Terminal after clicking on Log Out, no delay at all.