Windows 10 A lot of cpu is being used, but opening the task manager drastically reduces it. Why?

[Heera]

I have an Intel i7, and at times I can hear my laptop doing a lot work. I open the task manager to see which app uses so much cpu, and I'd see the cpu usage would be 100%, and 5 seconds later it'd drop down to 20%.
I'm pretty sure it's not a visual bug in the task manager because I can literally hear my laptop go completely silent after doing that. I've been observing this for the past 20 days or something.
Why is this happening? What can I do to keep the cpu usage at that low% instead of maxing out at 100%?

 

[Luke13TH]

This sounds like an OS issue..... first thing i would try is looking at msconfig, see what type of start up selection you have, reset it to normal start up if possible then restart..see if that helps. If that doesn't help, I would give the OS a reinstall and see how it gets on after that! Troubleshooting CPU issues can be very difficult and sometimes is quite time consuming. Back up your data, reinstall the OS and then take readings, I reckon a clean install will solve this issue if msconfig doesn't

 

[Neemobeer]

Actually this could be an optimization task. There are a number of tasks that only run when the system is idle or not under load.

 

[WindowsPro]

I would check for malwares, this sounds like you have some sort of coin-miner in your system..
Please check out: Free Security Tools for Windows, Mac, Android | Free Firewall and Linux AV Security Tool Downloads | Sophos
And download and run the free tools: HitmanPro - Malware Removal Tool and the: Virus Removal Tool

If you dont find any malware, there might be issues with your Windows installation..