Every instance mentioned here, when considered individually, might slow your computer down by only small increments. For example, using a pack of custom icons for a handful of shortcuts will not reduce your computer's speed to a crawl. It's the combination of all these instances that will have a significant effect.
A conservative approach will help in avoiding a sluggish system. Maintaining an organized desktop free of extraneous icons and gadgets, keeping taskbar items to a minimum, and uninstalling redundant or unused programs in addition to routinely performing general maintenance tasks will keep your system running smoothly.