I did a test of the control and video transmission lag time comparing the Spark RC connected to an iPhone 6 using the OTG cable vs. WiFi.

Test setup:

Spark & RC using latest public release firmware (not beta)

RC positioned about 12” away from the Spark

RC connected to iPhone 6 running latest public release of DJI GO 4 app

Recorded video with both the Spark and the RC/Phone in frame using another camera recording at 60fps and sent the gimbal up/down command 20 times each. I then frame step through the video and measure the number of frames between the recorded beep of the RC button press to the first frame where the Spark gimbal starts to move (this is the control lag), and the number of frames between the gimbal starting to move until the video downlink on the iPhone shows the gimbal movement (this is the video lag).

I won’t be posting the test videos publicly due to the immense mess in my work room. I will consider sharing the videos privately if anyone strongly desires to verify my raw data.

WiFi:

Control lag: min = 183ms, max = 317ms, avg = 277ms

Video lag: min = 233ms, max = 267ms, avg = 242ms

Total perceived lag: 519ms

OTG:

Control lag: min = 217ms, max = 367ms, avg = 300ms

Video lag min: = 150ms, max = 183ms, avg = 160ms

Total perceived lag: 460ms

Conclusion:

OTG video lag is better by 82ms but also suffers more control lag by 23ms, canceling out some of the improvement.

I must say that in both cases, the total perceived lag is disappointingly high, I would have expected less and I hope DJI can make a noticeable improvement in future updates. Note these results are specific to my setup and may be different for yours, especially using different phones (iPhone vs Android) and beta or future DJI software updates.

So, what is your advise? Should I buy the cable or not? Somewhere else it's mentioned the OTG option is not fully tested by DJI

I too feel that the lag and signal freeze is unacceptability high. When I'm in sport mode and run it out 1400 feet or so, I often have to just let off the sticks until video resumes to make sure I'm not running towards something at 35mph. Lol. I have tried wifi and OTG. The one thing I have learned is, when switching batteries, shut down everything. Force close the go 4 app. And start fresh with every battery change. This does seem to help reduce video freeze ups and studder.