KDE plasmashell goes nuts with network-heavy programs

[IsmoHaa]

I held out for the longest time refusing to update my KDE4 desktop, still being traumatized after the KDE3->4 transition, but a few weeks ago I caved and installed kde-plasma/plasma-desktop-5.8.5-r2. The problem I have is that occasionally the plasmashell process slowly goes bonkers and starts hogging one full processor core and then some. (According to top, CPU usage varies between 80 and 125 %)

I've managed to isolate the problem, enough to notice that it seems to occur only when running network-intensive programs, e.g. ktorrent or retroshare. Killing the plasmashell process and restarting it initially brings the usage down to reasonable levels, but within a few minutes its back to 40%, and given some more time it creeps back to the 100+ range. Needless to say that this makes the computer very sluggish and unresponsive - downright unusable for even slightly heavier tasks. Stopping the network-heavy application quickly brings usage back to a normal usage level of ~10%, but that's not really an acceptable solution.

Googling around reveals that I'm not alone with the problem of plasmashell running amok, but in all the other cases I've found, some desktop widget has been the culprit. I don't have any desktop widgets beyond a small number of icons to start frequently used programs with, so that seems an unlikely cause - they are of course also present when plasmashell is not going berserk. I also have a pretty bog standard panel, so again that's an unlikely source for problems.

Any suggestions? I like KDE, but I'm getting pretty f***ing tired of them breaking the most basic things with every major release, and taking ages to get the quality up to usable levels. (KDE 4 was horrible for dual monitor users all the way up to 4.7, WTF?!)

[asturm]

Certainly not normal, log out of your session and remove ~/.cache, maybe it helps.
_________________
backend.cpp:92:2: warning: #warning TODO - this error message is about as useful as a cooling unit in the arctic

[IsmoHaa]

[Hu]

If plasmashell is running wild, why not kill it or log out (thereby implicitly killing it)? A reboot should not be required. In extreme cases, it might ignore regular signals, but nothing can block SIGKILL.[1]

[1] Except buggy kernel modules.