With all the mentions of antivirus software:
When you’re in task manager see how many different antivirus programs are running. Running more than one can clog up a CPU.
I’ve found many of the “scanners” like to add their real time antivirus package to be “helpful”.
Large disk drives can get pretty fragmented over time and slow down the response time. It's the first thing I check whenever my Firefox browser (and other things) seem to be bogging down.
Had to show the wife that she needed it (defrag) as well, as she had the same problem.
Auslogic free defrag program also will visually present just how defragmented you disk is when you run it.
Good luck, whatever you find it to be.
bkmk
Im not sure if anyone else suggested running chkdsk but often bad file descriptors can add to this problem. Delete all temp files and clear browser history first. In Win 10 you can right click my computer, right click the os drive (usually C:) select the tools tab and go from there.
see what programs are running all the time — there may be many programs/services running in the background that you rarely use (change them from ‘automatic’ startup to ‘manual’)
if you can turn off animations/autoplay in the browser and try to use the older (simpler) interfaces on various websites
use ad blocking features - definitely ... the load keeps getting bigger as they cram more complex crap on the pages
bkmk
Have you tried overclocking the cpu?
It could be the hard drive failing or it could be something that happened to me.
My tower was running at a glacial pace. I took it in. They checked it out and it ran fine. I brought it home. The ice age returned. Took it back. They showed it to me and it was running great. Took it back home. Mammoths began to die.
I had a stupid idea. What if it were the power strip? Changed the power strip. The tower ran great.
So try plugging it in somewhere else with a different power strip. It can't hurt.