As explained in this other answer:
These symptoms sound like a CPU throttling issue that I had. When my laptop overheated, the CPU would reduce its speed (throttle back) to reduce heat output... as a result my CPU % appeared to be low at around 15%, but in fact the CPU had cycled back to 16% of its max and so it was in fact running flat out.
You can diagnose this by using Resource Monitor instead of Task Manager. (Type "resmon" into the Start-Run prompt.) At the top of the CPU tab processes window there is a useful "Maximum frequency" which will reduce as the CPU throttles back.
If this is the problem then you can look at causes of the throttling. You can use free tools like Speedfan to monitor temperature and check on your fans.
In my case there was a definite correlation between throttling and temperature. My laptop was a Dell E6400 and I used a fan assisted cooling tray to assist with the overheating, and I also hit a known issue with this laptop model - see
Run Windows Memory Test: Start->Control Panel->type "memory"->Diagnose your computer's memory problems. That will run the Windows memory diagnostic and it's a good place to start.
Run Command Prompt as administrator and then type "chkdsk /r" and hit enter. Follow instructions. That will check the integrity of your install.
If you had extra RAM lying around you could easily switch that out and see if it helped. It's possible that your video card is maybe acting up but that's just me trying to think of all the conceivable issues.