CPU usage is too high with Catalina when macbook-pro connected to an external monitor
From your observations it sounds like you have a temperature problem with your MacBook Pro. The system makes the CPU sleep when it detects a temperature problem - this what makes it appear as if kernel_task consumes all the CPU time (it actually doesn't). Usually this is due to either: (a) fan problems, (b) blocked vents or (c) lack of cooling paste. 

I would check these to make sure that fans are working properly, the machine is not full of dust, the vents are free and cooling paste has been applied properly to the CPU.