That wouldn't impact your performance much at all, as long as the application you have open in the second monitor isn't very demanding. For example, I used to run ARK on one screen, and Chrome on my other, and the second screen didn't cause a noticeable difference in performance.
However when I got a bunch of tabs open, or watched a youtube video or some more demanding application, it would impact the performance of the game I had up on the other monitor.