I did some unofficial benchmarks in KDE Buster. I installed the full metapackage and also installed a couple of plasmoids and redshift.
My hardware on this computer is as follows:
Intel i5-3320M CPU@2.60GHZ
6 GiB RAM
Test method: Fresh boot, opened Konsole to test CPU and RAM usage. Waited 30 seconds to read the results.
KDE with Kwin, compositing on: 448 MB RAM, 1% CPU.
KDE with Kwin, compositing off: 425 MB RAM, 0,3% CPU.
Turning compositing off has negligible results on RAM and CPU usage.
I then installed the openbox-kde-session package to replace Kwin with Openbox
KDE with Openbox, compositing on (Compton): 414 MB RAM, 0,6% CPU.
KDE with Openbox, compositing off: 405 MB RAM, 0,5% CPU
The difference between Openbox and Kwin with compositing off were a mere 20 megabytes in RAM usage, while Kwin used slightly less CPU. I'd call it a draw. With compositing on, Openbox with Compton used 34 MB less RAM and slightly less CPU, making it an option for low end computers.
Edit: Disabling compositing plugins in Kwin brought the RAM usage down to 437, while CPU usage remained unchanged.
Lenovo ThinkPad T440S, Intel Core i7-4600U CPU @ 2.10GHz, 8 GB RAM, 256 GB SSD, Debian Bullseye (KDE)
Lenovo ThinkPad X240, Intel Core i5-4300U CPU @ 2.90GHz, 8 GB RAM, 120 GB SSD, Debian Bullseye (KDE)