DO I need to do it after putting it on a cooler? (it's my first time) ... My warm up time is just 20 sec as of now...

Do i need to cal IMU ? some people said they did not cal and they have no issue.

THE DJI procedure also did not mention about IMU cal after FW update...

Pls advise..

My warm-up time is significantly longer after the update. I tried calibrating the IMU in a colder environment with not luck.

Follow-up:  I recalibrated on a much cooler day and I now have little to no warm up.

It's a good idea to do steps #47 and #48 at the following link after installing new firmware:
http://www.phantompilots.com/threads/how-to-install-phantom-3-firmware-if-using-dji-go.50065/

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Loaded new firmware v 1.3.20 then calibrated my IMU on the patio with temperature about 70F.  Completed RC and gimbal calibration then compass calibration at the field I fly out of.  My DJI3A IMU use to take a while to warm up before the new firmware and calibration, now its ready to go in seconds.  All is good.