I would imagine that you have a few icons on your desktop. Maybe more than a few. Maybe a lot. Turns out that “a lot” is bad. The problem is that the Finder (which displays the icons) is not very good at dealing with a large number of icons at once. The effect is your machine slows down, just a little, for every icon on the desktop. Incredible, but true. So, if you want your machine to be fast again, reduce the number of items on your desktop. It’s OK to put a couple of folders onto your desktop, and to toss zillions of items into those folders, because when the Finder looks at the desktop it will only “see” the folders (not what’s inside).