I encounter plasma becoming unresponsive as plasmashell CPU usage grows to 100%. All input will then become very slow and the screen/plasma may start to flicker. I did not wait for it to crash, but I had to force reboot the system in order to commence work.

The problem can be worked around by issuing 
kquitapp5 plasmashell; kstart plasmashell
whenever the CPU usage of plasnmashell grows to unusual values.

I am running KDE Neon user edition with all current updates and encounter the bug since a two-or-three plasma versions (since about half a year).

I cannot say definitely what causes the bug despite I am running the same variant of KDE on two very similar notebooks, Lenovo Thinkpad T520 and T530 respectively. Only on latter the bug occurs. 

One major difference is that I use it for work and regularly connect it to external monitors. Indeed, it seems that connecting or deconnectiong a monitor (which will cause plasma to run on both screens / only a single screen, with my settings) triggers the issue. (Another difference is that I use the T520 with discrete nvidia graphics while the T530, having the problem, features only an on-chip Intel graphics card.)

When I work only on the notebooks native screen, the issue does not appear. To automate the work-around, I described above, I have a custom script register the kquitapp5 plasmashell; kstart plasmashell commands to the event of screen-unlocking, using dbus. With this solution, I do in practice not encounter the problem of high CPU usage at all. However, I see the task bar building up again, each time I unlock, which is not what I would want.

There is a few related bugs (e.g. https://bugs.kde.org/show_bug.cgi?id=312919 and https://bugs.kde.org/show_bug.cgi?id=356479), but I did not see the problem being related to the attachment of external screens.