I'm an owner of a macbook pro retina 15' late 2013 that I bought (in used conditions) from the web 2 months ago.
Just few days ago, while having 2 external display and a thunderbold blackmagic ministudio recorder connected, the battery dropped to 0% despite the ac adapter was plugged in.
In addiction to this, sometimes the mac ran very slow (for example while hovering the icons on the dock, the magnifier effect looked jerky) and the kernel_task process took over 200% of cpu usage.
After that I found out the ac charger was 65W instead of being 85W (that was supposed to be included in the box... so it means I was mislead in first place from by seller). When I replaced it with the correct power adapter I solved all my problems.
But the doubt remains..
Does it make sense that the slow performance and the kernel_task process locking the cpu was caused by the wrong ac adapter only? Can someone confirm that I ultimately solved my problems using a more powerful power adapter? Someone who maybe encountered the same exact scenario and had the same issues. I ask because I'm afraid the cause was more subtle and I have still no idea if it could arise again also now I set up a more appropriate configuration.