First - Check if one of the processes is using 99% CPU or excessive IO . if not - try SMC reset :   


SMC reset instructions for a MacBook Air are:

  1. Shutdown your MacBook Air (go to the Apple menu and choose “Shut Down…”).
  2. Make sure your MacBook Air has external power. Connect the power adaptor, if it isn’t already.
  3. Using the Air’s built-in keyboard (not a plug in keyboard or bluetooth keyboard), of the left hand side of the keyboard press Control-Shift-Option and on the top right of the keyboard, the power key at the same time.
  4. Release all the keys at the same time.
  5. Press the power key to start the computer.