m15x with 24 inch external monitor lag

I just bought a 24 inch Dell ultrasharp 2408wfp, and am having some difficulties adjusting it to use my m15x with.
1. I am connecting it HDMI style (sucks thats the ONLY the the m15x has)
2. The problem is that when I treat the monitor as the main one in 1920x1200, and disable the laptop monitor (fn+f6), there is a tremendous amount of keyboard lag only. The mouse is completely fine, but the keyboard lags so that I have to type like 10wpm.
3. When I clone display (that is, both monitors are on and both resolutions are 1440x900), I can type perfectly fine and there is no lag.

So, does anyone know what settings or what I can do to adjust this so I can turn off the laptop monitor, only use the 24 inch dell at 1920x1200, and not have the horrendous keyboard lag?

Thanks in advance for any help.
-Andrew

 

One more thing:
When I do it so that the 24 inch is the "main" one at 1920x1200 and i choose "extend the desktop onto this monitor," it works perfectly (basically dual monitoring with my laptop not really being used)

But how do I make it so basically the laptop is treated as a desktop and the 24 inch is the only one being used, without the nasty keyboard lag?
Thanks!