How do I get my computer to rub faster?

Every time I play a video on my computer the CPU Usage goes out the roof and my computer starts running very slow. It requires multiple restarts before it gets back up to the correct fast speeds. I am just lost i have tried Disk Optimization, Scanning and cleaning all possible threats, cleaned all out all cookies and cache. I am just at a dead end here I do not want to have to pay Geek squad to fix this -_-!