External monitor causes very high CPU usage
There has been some unconfirmed reports that the NVIDA drivers that came with High Sierra are not operating correctly.

Suggestion is to get updated Drivers from NVIDA directly.

Some have suggested to change the resolution back and fore to fix the problem.

It also looks like your system is no doing Graphics switching.

Check in System Preference that the Auto Graphic switching is turned on.

Also it could be very useful to us if you copy/paste some 30-50 lines in from your Console at the time when you plug in the external monitor so we can see what is going on.