How about posting a Speccy so they can help you out
Why does my gaming FPS decrease by a good amount when I'm browsing on my second monitor?
Is it because of low RAM? I have 8gb
Is it because of my CPU? I5-7600K
Why is this and how can I fix it so that my performance isn't affected or atleast to the point where I can't tell a difference?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic