Speed up your Mac by removing files from the Desktop - Mac OS X Hints: "A family member owns a Mac, and he was complaining it was getting slow - especially the Desktop. He had a lot of files on the Desktop, but none of them were visible on the Desktop itself, but only in a Finder window displaying the contents of the Desktop. The Desktop only showed two icons: the main hard disk, and a Time Machine hard disk. The Desktop itself was very slow, and often displayed a beachball.
However, when I looked in the Desktop folder, there were more than 2,700 image files. When I removed the image files from the Desktop folder, everything went back to normal.
So, don't over-clutter your Desktop, as it might slow down your Mac and even hide files. "