Perhaps the unusually slow, with frequent beach balls the little colorful loop that pops up when the computer needs to process something before it can listen to you again .
Or is it the network that do not seem to be as quick as it used to?
Maybe you have problems with any program that refuses to quit?
When someone feels wrong with the Mac is it's time to start the activity monitor which you will find in the folder Applications - > Utilities.
When the computer beach balls or is generally lethargic
Few things are as annoying as having to wait on the computer when the to think.