It isn't unusual for a laptop to do that when charging. Not saying it has to do it, just that it doesn't mean there is anything wrong.
You should check your power settings and see if it is set to use less/do less when charging. That is often the cause.
Go into "Control Panel" and then "Power Options". Click "Change plan settings" Next to the plan you are using when it is plugged in and charging. On the next screen click "Change advanced power settings".
You will get a new window and in there search through the settings to see if it is set up to run on a lower setting when charging. If so, change it to the settings or plan that you wish. When you are done click the "Apply" button and then the "OK" button.