Macbook Pro very slow on Dual monitor setup?

Kernel_task running at an artificially elevated CPU level is the OS's method of forcing the fans to speed up to cool the machine; the side effect is to slow the machine down too, which aids the cooling process.

It can be sign of a hardware issue, but in the case of running 2 external monitors would probably be expected.

You can try resetting the SMC, just in case, but the symptom in itself isn't a sign there is anything wrong.

It turned out it's Macs fan control app that I had installed. For some reason, it's not working properly. I turned it off when the kernal_task's CPU went crazy again and it all worked fine after that.