I've upgrade to 16.04.1 from 14.04. Everything seems to be working, but some of the Unity hotkeys don't react for more than 20s, i.e. Ctrl-Alt-t opens a terminal after 20-30s, Ctrl-Alt-Del logs me out after >15s without any feedback window.
Other hotkeys like the Calculator or Mail Button work without problems. I tried to map the 'Launch terminal' to the Calculator button, but it was not faster. I mapped the gnome-calculator to another button, and it had then the delay, too.
A completely fresh test-user account has the same problems. The guest users hotkeys work without delay. I reinstalled unity and ubuntu desktop components without any change.
The virtual terminals Ctrl-Alt-F1 works without any delays.
My computer is a laptop with mainly Intel components (Core i5 Sandy bridge, with Intel HD3000 graphics).
I'm out of ideas now. I don't even know where to look further. I went through the troubleshooting guide at https://wiki.ubuntu.com/Hotkeys/Troubleshooting, without luck.