I recall that the kernel_task problem is actually a feature to protect the computer, I believe from overheating but possibly other things (not confirmed). Cleaning out the dust in the computer/fans can help.
I also found some luck by moving my external monitor to the second thunderbolt port. Possibly the one it was using is defective? Just to test, I plugged in another thunderbolt peripheral into the suspect port and the kernel_task problem did recur. So maybe that is the main problem, a bad thunderbolt port.
My setup is also mid-2015 MBP 15” and this occurs primarily if I use both the screen on my MBP at the same time as the external display (Dell 24” 2K) (i.e. so I have two screens).
Peace to all.