Get a faster computer. No idea how to help you when you don't post the specs of your machine or even what operating system you're on. More RAM would probably help.
How much space you've used is pretty much completely irrelevant. Tell us more detail and maybe we can help. Specifically, what exact model computer, how much RAM, what OS, what apps you are using, the exact nature of the problem, and anything you have done recently (or around the time the problem started). Just saying that your scrolling is lagging is not enough information for anyone to give you even an educated guess.