Try checking Activity Monitor (Applications>Utilities) to see if one app or process is slowing down your Mac.
There's more tips here. It's an older article but worth reading.
According to Apple's Human Interface Guidelines, "the spinning wait cursor is displayed automatically by the window server when an application cannot handle all of the events it receives. If an application does not respond for about 2 to 4 seconds, the spinning wait cursor appears." (WindowServer is the background process that runs the Mac OS X graphical user interface.) Which is to say, the beachball is there to tell you your Mac is too busy with a task to respond normally.
The beach ball may also appear if you don’t have enough RAM.
You don’t have to keep an Activity Monitor window open all the time; there are less obtrusive ways to use it. For example, open Activity Monitor then Control-click on its Dock icon and select Dock Icon -> Show CPU Usage. That will turn the icon itself into a CPU usage graph; you can then close the main Activity Monitor window. You can also Control-click on the icon and select Monitors -> CPU Usage, or Monitors -> Floating CPU Window. That will place a small activity graph in the corner of your screen.