This Asus model appears to support hybrid graphics (I can't confirm from looking at the Asus website):http://www.asus.com/Notebooks/Multimedia_Entertainment/N55SF/#overview
This is a power saving feature that lets you use the laptop's on-board Intel video adapter when less demanding tasks are running. When more demanding video tasks are running, the system will automatically switch to the Nvidia video adapter.
If your system is not switching back and forth, it could be part of the issue. I am only theorizing at this point. Try this, look at your BIOS settings and see if you can disable the on-board video adapter (Intel) and then test your graphics speeds.
Also, just as an aside, how does the system perform when you display to both the internal LCD and the external display at the same time?