To understand why processors, memory and other components of your computer slow down as they heat up you first need to understand the nature of electrical conductors. Electricity is essentially the transfer of electrons from one pole (negative) to another (positive).
When materials such as the copper in circuits heat up, their resistance increases, and increased resistance means that it is essentially more "difficult" for an electron to pass from one pole to the other. This is because as a material gets hotter, the atoms and molecules in it become excited, and this increased "jostling" means a greater likelihood that an electron traveling between the two poles will collide.
What this means for your processor and all the other parts of your computer that heat up is that it takes slightly longer for the signal from one terminal (such as a bit in memory) to travel to the other, ie. slower data transfer, slower addition/subtraction, slower everything! The cooler the better, to the point where nearing absolute zero you end up with things like superconductors that transfer those signals almost perfectly.