plasma can be a somewhat heavy DE. But after changing a few settings, it can be incredibly lightweight (only for CPU usage though. For RAM usage, it even surpasses gnome for some reason...).
From my testing, it was the only DE that made my old laptop be able to turn off the fans sometimes while using it. Not even xfce nor cinnamon were able to do that. Only LXQT and Plasma.
One DT is Zorin 17.2 running KDE 5.27.11. The other DT is on Lenovo Yoga 7X Arm64 running 24.11 xle Ubuntu Kernel with the kde DT installed at the kubuntu 24.11setting giving me kde 6.1.5- yes!
The lightest Plasma of 5.27 you will find is the default PCLinuxOS version. I am not saying lighter than xfce, but compared with other distros implementation, and I believe it is down to not implementing Discover, all software installation and updates are done via Synaptic Package Manager which appears on the Panel by default. I've tried their xfce offering but could not get printer installed when looking for a replacement for MX Linux on my lady's machine.
This is what Plasma on PCLinuxOS uses in memory when idle: