Any, "recognised" speed run has to be set up with a fully SURVEYED distance of the course to sub millimetre accuracy, using "light beams" (generally laser) set at 90 degrees across the course at both the start and finish of the "run". These beams are connected to a very precise timing device that is automatically activated when the vessel “breaks” them.
Why do you think that the organizations that are responsible for determining the accuracy of "official", so called "speed records" don't just take the easy way out that is being promoted here, and just use a GPS reading?
Do you imagine for one minute that the GPS system hasn't been fully investigated for this purpose? Fair suck of the sausage, those people are professionals and their results have to be "beyond reproach".
If you want a "comparative" speed reading over some distance or other, then, by all means, use a GPS reading, but it will not be anywhere near accurate enough to rate against any "official" record, regardless of how hard anyone "pushes that barrow". Any GPS readings, especially for distance covered over time, are just too “subjective” and fall down badly when objectively analysed.
It can very easily be shown also, that the “longer the distance recorded, the slower the average will be” that is why it was agreed many years ago that the “standard” run for recording “speed records” for sailing craft, would be over a distance of 500 metres.
(If history is not known then the human race is destined make the same mistakes “ad fin idem”)