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?
How many windows or tabs do you leave active in a web browser while browsing the web?
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?
Look at what is happenning with your Mac when you run Activity Monitor.
Macintosh-HD -> Applications -> Utilities -> Activity Monitor
Click on the CPU tab on the lower half of the window to see how much time you are using.
Click on System Memory to get statistics on memory usage. You should have some free memory.
When you have lots of page outs more memory will be helpful. My entry for entry for Page ins/outs is:
Page ins/outs: 29163/0
Notice I have 0 pageouts which says that I am not using my harddrive for extra memory space. Thus, I have enough memory.
Activity Monitor has a neat feature where it can display a dynamic dock icon. In Activity Monitor View > Dock icon > Show cpu history.
"Reading system memory usage in Activity Monitor :"
Problems from insufficient RAM and free hard disk space