Since upgrading from libQt5Core5 5.9.0 to 5.9.1, I experience two issues with
plasmashell, which seem to be caused by the same core problem.

The first is that the desktop causes massive memory usage. In over an hour, my
system goes from using 2GB of memory (after booting and opening my
applications) to approximately 8GB! However this memory is not attributed to
any one process; It seems to be disk cache memory, which is further indicated
by the fact that issuing the command "echo 3 > /proc/sys/vm/drop_caches" causes
some of it to be freed up (not all).

The second issue is that every few hours, plasmashell will crash and restart. I
use an image slideshow wallpaper, and before the crash it always becomes black
instead of an image... plasmashell then dies immediately after I attempt to
click anything on the desktop.