And I’ve run KDE Plasma on a lot of hardware, a lot of it very old, and it’s been fine, if with slightly slow loading times (I daily drove that single-core potato I mentioned for about a year on Plasma).
I’m very sorry it felt sluggish for you but that’s likely down to your specific hardware configuration, drivers, GPU vendor + display server combo, etc. Plasma is not that bad for most people. You just got unlucky.
EDIT: Actually, if you actually somehow installed 5.6 on modern Debian with modern Qt frameworks etc, that could be why it was so slow. Could have been a fucked install.