Macbook slows down when connected to external monitor and power supply

I have a Macbook Pro (Retina, Mid 2012) with the following configuration: 2.3 GHz Quad-Core Intel Core i7, 8 GB 1600 MHz DDR3, Intel HD Graphics 4000 1536 MB

I use Dell P2419H as my external monitor which I connect my macbook via HDMI cable. I have calibrated the external display profile using the settings > display. The issue is when my macbook is being charged and connected to the monitor it becomes excruciatingly slow to the point of not being of any use.

This does not happen when I am running my macbook on battery power and connect to the monitor.

In both the cases my macbook screen is open so the normal airflow is not hindered in any ways.

When I see my activity monitor it seems that on plugged in mode power the kernal task increases.

Here is a screenshot of activity monitor when laptop is running on batter and connected to monitor

Here is how it looks when on plugged in power + external monitor