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:
- Shutdown your MacBook Air (go to the Apple menu and choose “Shut Down…”).
- Make sure your MacBook Air has external power. Connect the power adaptor, if it isn’t already.
- 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.
- Release all the keys at the same time.
- Press the power key to start the computer.