Apparently, Apple has programmed kerneltask to grab available CPU resources when it determines that the CPU might overheat due to other tasks stressing the CPU too much. In my case though, there was no danger of overheating (running somewhere between 55 and 60 degrees), but kerneltask just reacted to my 4K monitor being plugged in in addition to me using the internal laptop retina display. As soon as I would unplug the monitor, kerneltask would release CPU resources and go back to < 10% !!
So basically, shitty programming on Apple's part had rendered my laptop unusable for quite a while.
Luckily, I found this article which worked in my case also: https://www.davidschlachter.com/misc/kernel_task
My laptop is now a joy to use again, but I can't believe that something like this remains unfixed for so long. I've seen many people on message boards running High Sierra with the same problem, so this just doesn't seem to get fixed.