Let it cool. Don't fly it for an hour so. Find a cool place and a levelled surface.
Then do the calibration immediately the moment you turn on your phantom.
The cooler the phantom, the faster the calibration and the more accurate it is.
The biggest mistake is when users ****-around in the GO app and then decide to do the IMU. By this time, the inside of the phantom is very warm. And the warmer the Phantom, the longer the calibration will take.
If you are in a rush, pop the battery out and stick it near an air conditioner. DO NOT place it in the fridge like others have suggested. This may cause condensation in the camera and circuit boards.