It's likely to be 198ms + the buffer speed. Just saying this because I read where someone was using an electronic flash off a P4, and an
I2 with a X5S and a X4S camera, and posted the sync speeds from hitting the shutter button on the RC unit (Not GO 4) to fire a flash in conjunction with the camera shutter in the DJI forum (Bookmarked it here:
Using studio flash on a drone. ).
Time to write (Buffer speed) to the SD card will vary depending on the card too. I know I have a 64GB Sandisk with a write speed of 95 and 300, and the 300 speed UHS-II card is much faster to dump the images into on the Olympus camera I use. I doubt if DJI cameras run at top speed though.
My WAG would be around 0.215+ seconds for a starter.