Since nobody answered, I'll try to help, although I don't use neither debian nor KDE.
First, debian uses a very strict release policy, which means you're probably running ancient versions of both STK and KDE. It could very well be that your issue has already been fixed, it's just debian isn't shipping the latest versions. Try dist-upgrading to
sid, which contains newer versions of the packages (don't let the name frighten you, it isn't unstable at all.) Alternatively do what I did, ditch debian and switch to a rolling release distro. As far as I know, Fedora, Manjaro and openSUSE Tumbleweed all ships KDE out-of-the-box.
Second, try ditching KDE. It is a bloated and resource heavy monster, but it could be hard to find an alternative that suits all of your needs. I won't recommend anything instead, because choosing a desktop environment is mostly a choice of personal taste (FYI I don't use any DE either, I've created my own .xsessionrc to run a compositor and a minimal wm and that's it).
Cheers,
bzt