I doubt that it would have taken up to 30 secs to get a stabilised GPS speed. Experiments with this unit on the ground, where a weaker signal is more likely, suggest a stabilised speed can be achieved in a few seconds. Also the test procedure was for the pilot approaching Vs to continuously call out the IAS to the observer, who compared this to the GPS and took notes. If the GPS speed was dropping when the IAS was stable just above Vs, the test conditions would not have stabilised and the readings would obviously not be a valid comparison (i.e. obvious to the observer). Having said that, I cannot prove now that there was no GPS lag and the test is worth repeating.
If the stalling speed (TAS or CAS) were really 55 mph, then the approach speed should be over 70 mph, which does not seem credible for this aircraft.
No graph of IAS v. CAS is available, creation of this would be a useful exercise. I need a day of stable air, with low wind speed, not so easy to get these days!