Lenovo Y570 - Better performance when outputting to an exteal monitor

To make this simple: When I try to play Overwatch (or any game, using Overwatch as the example) on my Lenovo Y570 on its own LCD, it averages about 30 FPS, with the sound quality going down with it, until the sound is audibly slideshow-ing.

However, when I hook up a Lenovo P2770HD laptop and output my video there, I easily hover around 60 FPS. What interests me most is that the resolution increases, so that's not an issue (1366x768 -> 1920x1080). Completely dumbfounded as to what could possibly cause this.

All the info you need to know about my computer: http://pastebin.com/Prs6DZSq

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