windows responding slowly....what's happening?

Not pretty sure the config of my PC but it's intel i5 2th or 3th generation, 8GB RAM, 128GB SSD.

The CPU has been used for 6+ years. SSD and RAM were new in 2016, currently running windows 10 Home.

I don't install or run any games on my PC. Windows update is done from time to time.

Recently the whole thing getting slow. For example, click on a windows it does not show up immediately, stopping or resuming a youtube video could delay a few seconds, which is absolutely not normal

I've checked the mouse. It should be fine. It's one-year-old Microsoft wireless mouse. Dust on the lens is regularly cleaned up.

What's happening?

Anything I can do to test if there are any problems.

EdTittel said:

For starters, fire up Task Manager and click the Details tab, then click the CPU column so it shows the high-end consumers of that resource. Take a screenshot and post here so we can all see what's going on, like so:

Hopefully, this will get us pointed in the right direction to help you get your system back on track. Be prepared: this can take a while and may require lots of back-n-forth posting and screenshots.
HTH,
--Ed--

PS: Just saw this in the very next thread I read: Fall creators upgrade messed up my system - Page 4 - Windows 10 Forums. It explains a couple of simple registry hacks that some users have found helpful with the slow response situation.

Could it be due to the CPU problem?
In fact, I bought the CPU 6 years ago in a proved dishonest shop.
Could it be a fake or failing CPU