Just updated to the LATEST *STABLE* release (10.12) on my mb pro retina early 2013 that was working perfectly fine with 10.11. Now when I connect my external monitor kernel_task eats up 500% of CPU. Of course because of the genius apple upgrade system I cannot revert back.
The same problem is references in the BETA release here https://forums.developer.apple.com/thread/49163 ?? Must have enough time to test this BASIC functionality before releasing it no ?
What is wrong with you Apple ? Headphoneless smartphone and now this ?