I record my races on an iPhone 7.
If I replay the track on the phone it gives a much higher maximum speed (at least 2.5 knots) than the information given in the analysis once I’ve uploaded the data and set up the course.
There is no part of the track before or after the race when I could have been going 2.5 knots faster.
I also have a Garmin Vivoactive HR watch running RaceQs which shows the same (lower) speed when the track is uploaded.
The phone is kept below deck on a Weta Trimaran while tracking and there’s no source of interference. The app shows a GPS accuracy of 3-4 while the track is replaying.
I also notice that the Maximum speed is not picked up by the app unless you replay it in full at slow speed- ie it doesn’t appear to parse the data to deduce the maximum speed.
I’ve attached a screen grab from the phone App as well as the phone GPS file after uploading and from the Garmin watch.