You can easily analyze drives with CrystalDiskInfo, because it highlights critical values and you can check what those mean in Wikipedia.
You don’t have to be an expert to understand that a yellow or even red status bar in CDI with a high number of reallocated sectors in the raw values section means that the drive is working poorly and that you should replace it as soon as possible.
If the hard drive is faulty you can optimize as much as you want with clean program/Windows installations, defragmentations, the main problem won’t vanish unless you replace the drive and 4h for normal scanning is a major problem unless there are some full 4TB 5400 rpm drives with only a lot of small files in multiple archives while doing some other tasks on the computer.

Also maybe the OP cannot afford an SSD, I know I can't.
You don't need to to replace your HDD with a SSD of equal space which would be insanely costly if you wanted more than 500GB space, however, a small 120 - 128GB SSD with one 1-2TB or your old HDD for storage purposes doesn't cost the world.