You might check Task Manager to see if you can tell what is using CPU time.
The Resource Monitor or Performance monitor might also help.
Something I bumped into last week was to use the Powercfg utility to check you system. It might give you a readout of what is using CPU time. Open an Administrative command prompt and type Powercfg -energy and let it run. It is best to close down all active programs you can. Make note of where it puts the .html file and copy it to the desktop to open. If it states the CPU usage is high, it might tell you what is doing it.