Laptop GPUs are slower than their desktop counterparts sometimes a lot slower.
While doing some research I've found out that the Mobility HD 5870 is about as powerful as the desktop version of the HD 5770. If you compare the desktop version of the HD 5870 and HD 5770 you will notice the huge difference between those two models.
You need to remember that laptops have to use lower powered components otherwise they would draw too much power (unless you don't mind your laptop to last for less than 2 hours on battery) and there's heat to consider. You wouldn't want your laptop to melt while playing Crysis right?