Major game lag when laptop is NOT plugged into a monitor?

I've been having this problem lately. On some games, they run perfectly smooth when I have my laptop plugged into my monitor, but when the monitor is not plugged in, the games lag horribly. I can tell you for a FACT it is NOT my pc specs. Lenovo y510p dual Nvidia GTX 755m in sli and Intel i7 should not cause lag. Anyone know what this could possibly be? It only happens on some games. Garry's mod, team fortress, work fine but things like GTA4, ARMA 3, etc Lag horribly when not plugged into my monitor.