There can be many causes for a slow running computer- anything from too many things loading at startup, slow broadband, something wrongly hogging resources, etc.; right through to a malware infection.
CCleaner is a good place to start.
I would also recommend running scans with Malwarebytes3, Junkware Removal Tool, and AdwCleaner.
You can find more about, and download, those 3 here: https://www.malwarebytes.com/products/
Usual disclaimers - Whilst meant for users with little or no experience these things can still have risks - You should make a backup before using - I have no connection to Malwarebytes - Other, similar, products are available.
If your still having problems then take a look at the links in item 10 of this:
https://forum.piriform.com/index.php?showannouncement=15&f=4
At any one of those you will find people who will take you step by step through checking your computer and getting it running properly again.
They will ask you to run a few programmes and send them a few logs.
If you use their services then follow their instructions carefully.
Though primarily there to help remove malware they are quite happy to look for anything else that may be causing problems.
And they are nice people who give their help for free.
One thing, they are nice people, none of them will help anyone who has pirated software on their computer. (I'm not suggesting for one moment that you might have). It does happen now and again, they sometimes give one warning to remove it.