Performance Differences When Not Using a Second Monitor?

[PhysicistSarah]

I recently took my laptop out (Which is rare for me, usually I just use it on my desk) and have noticed a significant performance improvement for seemingly no reason other than the fact that I'm not using a second monitor.

Here is my hardware profile:


I've heard about this from other threads, but what causes this? Does it put a strain on the system that can't be compensated for? All of the windows feel very snappy, which is abnormal when I'm using the second monitor. DOes this have something to do with my specific graphics card NVIDIA 750M? Are there any workarounds to boost dual monitor performance?

[Teunis]

My W520 has two video cards, the Intel one only works on the internal monitor, as soon as I want to run a second (or external) monitor the nVidia card has to be used.
This card can heat up and cause the CPU to throttle down, switching off 3D does help to keep the temperature down