On older laptops you may start seeing this problem. It seems to be caused by the CPU going into a protective shutdown whereby it reduces the clock rate to a small amount (seen on a Celeron-based IBM R31 laptop).
In Task Manager (ctrl+alt+del) then you will see 100% CPU with taskmgr.exe consuming a lot of this.
The laptop will run very slow and if you can get CPU-Z running it will show up a low core speed.
The only temporary fix is to reboot (sometimes hard shutdown by push-and-hold the power button for 10 seconds) but if the machine has cooling problems then this is not going away and you’re going to have to investigate deeper into the machine.