Exactly. Actually, a few years ago, there was a twit by a XFCE dev claiming that KDE was in par with XFCE as far as system resources are concerned.
I believe that dalto has put it in its practical dimensions, in a discussion here:
There are (at least) 3 metrics to consider here in the forum:
- Memory Usage - In 2022, plasma and xfce are similar in memory usage with plasma being slightly lower for most users. It is not enough of a difference that most people would care.
- CPU/GPU usage - Out of the box, plasma has many more effects and animations than xfce. However, if you normalize these, they will be similar.
- Disk usage - Plasma is heavier on disk usage than xfce. If you have an hdd or other slow storage device, there will be a noticeable difference here.
That being said, they are very different so which one you prefer should also be factored.
*** Forgive me for being off topic. 