Some of Airtronic's speed claims are dubious at best. I heard some lab oscilloscope test found the 4px having 3ms refresh rates which is pretty fast enough(but don't quote me on that). I know it doesn't test the actual time between when you turn the wheel and when the receiver sends out the signal, but in my experience once it gets fast enough to a point you really can't tell anymore unless maybe you're a sponsored pro driver. When I switched from 3pm to 4pksr I can tell there is a difference in response time. However I personally can't tell the difference in speed between my new 4px and 4pksr even though Futaba claim 4PX is 30% faster. At least for me I feel the newest generation of radios as reached that threshold that faster won't make a noticeable difference anymore. But the improvement in material, trigger and wheel feel definitely makes a difference when driving. The physical response of our rc cars are probably much slower than that of our radio now to be honest.

My 2 cents is not to get too worried about the response time for the top of the line radios from big manufactures. The physical build quality and egronomics are more important. Try out all the radios in your hand and you will be able to tell the difference right away. Pick the one you feel most comfortable with and with the easiest manual layout for you is my suggestion.