Big Red L: might I suggest that you divulge a bit more info about your system as "Buttons" requested in his Sticky post at the top of this Forum.
You probably have a "leaky App". That is an application that progressively uses more and more RAM thereby slowing down the rest of the processes.
First thing to look at would be the latest new software that you recently installed. Delete it/them and reinstall one at a time to see which, if any, makes the system change.