I was having this issue and solved it by adjusting the refresh rate of the external monitor.
Go to System Preferences>Displays
Click Display Settings...
Select the external display
In my case it was originally set to 27 hertz. The only other option was 30 hertz. A soon as I switched to 30, the CPU usage dropped. I wouldn't have thought that increasing the refresh rate would ease the CPU usage, but it did.
2019 16" Macbook Pro
antun
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