Do you clean up the temp. files once in a while...if not, you could have a gazillion on your machine. Download "CCleaner" and see how ling it takes to run...it's free, and automatically runs on every startup.
I've also got an old computer (XP) that had a lot of your same problems. Took it into the shop and had them clean it up. Amazing all the old obsolete Microsoft programs and files that they got rid of...Got a lean,mean mchine...Only cost me $60.
As an example, my Malwarebytes full scan used to take about 21/2 hours, now, it only takes about 30 minutes to scan.