I'm also pretty much always plugged in. It's likely that the issue was manifesting before switching to a 4k monitor, but after switching the issue became unbearable. E.g I'd wait 2-3s to switch between windows or desktops. Running both screens on native resolutions and I'm switching between windows near-instantly.
Do you run your laptop on non—native resolution? Or an external monitor with a non-native resolution?
I'm runnign on 2018 MBP, 16GB RAM + 4k external monitor. I experienced the same type of issues a couple of months back — high WindowServer CPU, mega choppy UI after a few days of use. Initially thought it was Safari, but it kept happening with other browsers as well. Researched it a bit and found a thread where someone suggested running both the laptop and the monitor on native resolution. Haven't had any problems since doing that. WindowServer sits at about 10% CPU and ~2GB RAM, current uptime 10 days.
I'm also pretty much always plugged in. It's likely that the issue was manifesting before switching to a 4k monitor, but after switching the issue became unbearable. E.g I'd wait 2-3s to switch between windows or desktops. Running both screens on native resolutions and I'm switching between windows near-instantly.
¯\_(ツ)_/¯