Mac at 100% CPU Usage when plugged into monitor I just got this Asus 1080p monitor and I've connected my Mid-2014 13-inch rMBP (2.8GHz Intel Core i5, 16 GB of memory, Intel Iris 1536 MB Graphics, running El Capitan 10.11.6) to it via HDMI. The Macbook is in a vertical dock in closed clamshell mode. It's plugged in with the screen off, and I've got a cheap Anker keyboard and Magic Trackpad 2 connected to it. When it's plugged in via HDMI to the monitor, whether the monitor is on or off, the CPU load is constantly at 100% with the fan spinning rather loudly. 

Activity Monitor shows the CPU load is almost all from kernel_task, and ~3% user, ~97% system. 

When I unplug the HDMI cable, the fan stops going and CPU usage drops immensely. 

Is this behavior from my Macbook as unusual/harmful as I would think, and what can I do about it?

Edit: Google Chrome and Xcode are running significantly slower on the monitor than they do on my Macbook itself.

Edit 2: As soon as a posted this, my fan stopped whirring so rapidly (though CPU usage did not drop from 100), and Activity Monitor showed that CPU usage is about 15% each from 5 different Google Chrome Helpers (?) and only 1% from kernel_task. What on earth?
In the end, after I let Activity Monitor calculate things for a few minutes, it was Google Chrome Helpers causing the CPU load and not the connection to the monitor. I quit Google Chrome and killed its Helpers, then restarted Chrome about an hour later, and the problem fully subsided.

If the CPU load doesn't come from one of the apps, and really is only happening because of then monitor connection, see bmike's answer.