Did you try turning it on and off again?
Haha but in all seriousness, I had the same issue with my Macbook Pro. You can gain a lot of unnecessary clutter without even realizing it. Many times the slowing down is because of memory. If you don't want to download an application to clean up your computer of useless junk, try going through manually and deleting things you never use. Find applications and files that take up a lot of space but you don't use and remove them. Clear your browser caches. And returning to my first statement, a big mistake a lot of Macbook users do is never actually turning their laptop off but simply closing it. Restarting can actually do wonders if you haven't done so in a while.