What could cause a cumulative slowness that manifests itself as high WindowServer CPU usage in High Sierra?
I resolved this on the same system by installing the 10.13.2 supplemental update, and then the .104 version of the NVIDIA Driver Manager. (the installers are very picky about versions, so pick whichever matches your OS version here; nothing before .104 installed right for me https://www.tonymacx86.com/nvidia-drivers/ )

Restart, and then switch to the NVIDIA Web Driver from the manager:


(edit: driver helped for a while but the memory leak came back. sigh. back to not having scaled mode available on external screens...)