I've have a maxed out late 2013 MacBook Pro, 2.6GHz Core i7, 16GB of DDR3 RAM, 1TB SSD, NVIDIA GeForce GT 750M 2048 MB, Intel Iris Pro 1536 MB.
I also have a DELL P2415Q Display 24-inch (3840 x 2160) using Thunderbolt, speakers through the headphone jack, and use a USB to Thunderbolt to charge my phone.
I upgraded to macOS Sierra the day it came out, and had no issues until this morning. I unplugged my external display and speakers, and used my laptop for about 15 minutes off the battery, then returned to my desk and plugged my Thunderbolt display, and speakers back in, and my CPU shot up to 100% and never came back down. My Mac is essentially useless as kernel_task is eating all my CPU, and preventing any other application from grabbing CPU.
I've since done a hardware check - everything was fine, reset the NVRAM, and I attempted to clear the .plist files from IOPlatformPluginFamily.kext but I couldn't find it on my Mac (was trying to follow this guide, booted into Safe Mode and everything then went to try and find the file and my Mac just doesn't have it). When my external display is plugged in if I do anything CPU intensive (video calling, opening Chrome, etc) then my kernel_task goes crazy again, and the only way to get it drop is to unplug my display. Plugging my iPhone in to charge also spikes my kernel_task as well. I've been monitoring my internal temperatures, and nothing has been out of the ordinary, currently chilling at around 140°.
At this point, it seems that any time anything puts load on the system and increase internal temperatures of any internal component of my Mac the kernel_task maxes out my CPU bringing my Mac to its knees.
Any ideas how to remedy this? Is this just a major bug in 10.12?