Another problem: ever since I switched from #Awesomewm to #i3, several applications occasionally hanged (specifically: their user interface). No CPU spike or such was detected. It turns out that those apps were trying to send notifications. Awesomewm supports that out of the box. A quick fix:
systemctl --user start xfce4-notifyd
One thing I was not able to do with #i3 so far: with multiple displays, it is all too easy for the mouse cursor to accidentally cross the border to a different screen. I can think of two possible fixes:
1. Make the screen edges "sticky": you only cross them after going extra <n> pixels across the border.
2. Optionally prevent crossing screen borders altogether.
None of the two seems possible with i3.