What year, screen size, CPU speed and amount of RAM installed?
How full is your Mac's hard drive?
How many applications do you run simultaneously in the background while working in another application?
Do you run any antivirus software on your Mac? Antivirus software can slow down the normal operation of OS X.
Do you run any "crapware" like Mackeeper or any other type of so called hard drive "cleaning" apps?
Have you downloaded anything from the Internet recently that could be the cause of this slowdown?