Try blowing out any dust inside of it to help it stay cool. Take it outside on a sunny day and do this so that the dust doesn't end up inside. Also, there is a thin layer of what is called "thermal compound" between the CPU and the heat sink. This is supposed to fill in the gaps between the metal surfaces so that heat flows more freely, but over time it dries out and doesn't work as effectively. You can carefully clean off the surfaces of the CPU and the heat sink and then apply a thin layer of fresh thermal compound to help it cool more effectively. Modern CPUs will throttle themselves when they get hot to prevent damaging the hardware, so keeping them cool can help performance.