The editors at DigitalTrends did a small experiment to find out. You can read it here. They checked the temp of a laptop while running some processor-intensive programs while on a flat surface, and again while laying on a sleeping bag.
In a nutshell – no. Heat by itself will not slow down the laptop. What it will do is cause the laptop to fail completely. Having taken many laptop cases apart, I can safely say that they collect more material inside them than do desktops. This fouling is due in large part to the surface on which they are placed, as well as their having very small vents through which dirt may enter, but seldom departs. All that accumulated dirt will act as a blanket inside the case, preventing the cooling system from doing its job. Over time, the constant overheating can degrade the components until one or more fail. Then its time for a new laptop.
So my recommendation from here in Help Desk Labs – spend the $20-$30 on a cooling station and don’t bring your lappie to bed.