WindowServer High CPU using external monitor - Big Sur

So, few bits of context

• I've got a MacBook 12" from 2017
• It's connected to an external monitor via USB-C to a Satechi V2 dongle and then HDMI to the monitor

I'm experiencing a few things:

1. Stuttering when I move windows (like dropping frames)
2. Videos stutter when my mouse pointer is in motion
3. WindowServer in Activity Monitor shoots through the roof of CPU usage when I move my mouse a lot
4. The above is accompanied by the beach ball

See screenshot

I've had a bit of a speedup by disabling "Displays have seperate spaces" in the System Preferences, but it's still happening. It's also slowing down game animations as when my mouse it moving, games won't animate.

Any thoughts?

If you're using an usb mouse, I've noticed that my gaming mouse causes the WindowServer to use a lot of CPU.

Turning transparency down in accessibility settings has helped quite a bit.

As of Dec 9th, 2020, it looks like it's still happening intermittently with one 4K display and my 2019 Macbook Pro.

You’re using an unaccelerated virtual monitor. Because your external monitor has no GPU to accelerate its graphics, all rendering is done on the CPU for that display. You can speed it up somewhat by going to the accessibility system preferences and enabling the Reduce Transparency option. But it’s always going to be much slower than an accelerated display.

UPDATE: My original conclusion was pure coincidence.
The cause was Night Shift. I had Night Shift scheduled for sunrise - sunset. Each day at sunset while Night Shift was transitioning, for about 20 min, WindowServer would use a lot of CPU. It's a macOS bug. I disabled the schedule and do it manually when I need to. It's all been good since then.

INITIAL answer:

In my case, macOS Mojave on a MBP 13" 2015 connected to a qHD external monitor over HDMI. Logitech G305 gaming mouse.
Sometimes the fan spins up close to 4000 RPM and stays there even if I just use Finder and WindowServer shows high usage constantly.
The fix for me: disconnected the HDMI monitor, turned off the mouse, moved the mouse USB dongle to the other side, then reconnected the monitor and turned on the mouse. All was nice and quiet then.

My problems were completely identical with OP's, and my logs were showing the exact same results as well.

I tried many things to work this one out from my research on the Internet, but no luck.

In my case the issue was resolved by changing the refresh rate from 75Hz to 60Hz. I don't know the reason but it is magically decreased as soon as I did that. CPU usage is better than ever, no fan noise which was the most problematic issue for me. Also, changing Displays > Color > Display Profile from PHL245E1 to SD170M-A might have helped as well, but after I changed it back to HD one, no significant CPU increase or fan noise detected. Btw, I realized that the Color profiles have HD and SD modes, and in my case PHL245E1 profile is HD one while SD170M-A is, well, obviously the SD one. If someone has a 4K monitor and having issues like this, it might be of help checking these settings as well.