Generally speaking,
The reason why KDE (and Gnome) use so many resources is because by default so many “bells and whistles” are turned on.
If you disable graphic bling and unnecessary automated resources, you’ll probably also be able to decrease the amount used.
But,
As you’ve discovered, it can make more sense to start minimal and add what you want instead of starting with everything turned on and figuring out how to turn things off.
TSU