The modern PC all do this as they age go slower, and slower and cause #1 is: (That HDD are born to fail, and self heal, and to run slower as they age. That is there THING!) A new SSD is one cure.
This page below is not about the internet slow as seen here.
The below topics are to cover PC slow, just the PC alone.
Besides the Captain Obvious cures, like a newer faster PC, more DDR memory 8GB is better, and a super fast SSD boot drive upgrade.
These word HDD means Hard disk drive and for sure the Windows boot drive.
W10 actions on #9 above (Microsoft's built in A/V is very good, I've used it for 5+ years now, and zero problems) I also only use free Malwarebytes, to make it stronger , just that.
In the MS, Security pages, turn on auto scans. 1st, then do full scan now here. (A/V = Anti-Virus, MS = Microsoft)
Do quick then full, if wise. (in the start + run box type windows security)
If the above scan fails to find any Virus etc., then try these: (online A/V tests !)
There are many but ESET wins my award.
If all else fails reload the OS FRESH, as directed in you OEM PCs users manual under chapter "RECOVERY"
#5 Elaborations: CPU over loaded? (CPU or Memory) First is MS Systems Internal app.
I use this fabulous utility (autoruns64) from MS to find things I do not want on my PC. < a safe free and portable APP, no install at all !!
Pure bliss this app is. (no cloud or MS store needed to run it)!!! I run that and defeat most auto running apps, like Adobe for sure. (vast bloat-ware gets you vast auto update startup software, I kill all I see)
My auto runs are , only Fences and MalwareBytes.
Nobody with 100 apps loaded and needed, and with 100 auto-update engines in the startup folders, ever, but they are not listening to me. (but I don't care that I just nuke them out of there using the magical app above)
If all else fails, there is this great man, ask him for help. The late and great Johnny Carson (RIP)
Sorry for my poor jokes, but that makes life fun.
Jargon , (nobody wants to say central processing unit or Microsoft corporation, hard disk drive or solid state drive, endlessly.)
version 1. 12-12-2020 (only w10 supported by me, time to upgrade?) by C.Obvious