If you are getting the IMU Warning Up message for a decent amount of time when you are starting your Phantom or Inspire, follow these easy steps to reduce the warm up time.

The reason your IMU needs to warm up is when it was calibrated the starting temperature was higher then the current temperature of the computer.   Operating temp for the Phantom is around 50-60 degrees C.  If you did a calibration at that level the IMU will wait until it is at full operating temperature before you can fly because all of its flight sensor data is mapped to that temperature.  By lowering the temperature and recalibrating you get a more diverse sensor map and can start to fly earlier and at lower temperatures. 

To do this:

Take out the battery
Put your drone outside at night, on your air conditioning vent in your house or car, or last resort in your fridge. 
Wait about 10-15 minutes
Turn on the controller and get into to DJI go app on your device
Start the drone, and as soon as it connects preform an IMU calibration.

Now the sensor maps will include lower temperatures so the flight computer will be able to start flying at a lower initial temperature. No more IMU Warming Up message for minutes on end.