I've just installed F3 on my laptop, what's the most demanding area in the game? I really just want to test the limits of my laptop.

Are you sure you don't mean your GPU? The CPU typically works hardest where ever there are a number of NPCs, as it is processing all the AI routines they're engaged in. So places like Megaton and Rivet City would be the most demanding for that. Your GPU on the other hand is all about your graphics. In that case it would be around the Potomac, as rendering the water would be harder on the video card/chip than simply being out in the middle of the Wasteland where there's nothing but rocks and trees.