The speed recorded isn't calculated from distances; it's the real-time speed as returned by the GPS at the time the snapshot is taken. Most consumer GPS units use much cleverer stuff than just time-between-points to measure distances.

You can also drag the mini aircraft symbol along your flight in SkyDemon (in the virtual radar) to see your speed, altitude etc at all points.

Thanks Tim, but I'm not there yet.
I can see the track log on the map and on the virtual radar (on my PC), but how do I get a  'mini aircraft symbol' - I just have a standard white arrow pointer.
I'm obviously missing something!
Jim Harris

Jim, perhaps you could post a screenshot of what you're seeing. The white aircraft symbol that can be dragged around is in the virtual radar window. The documentation almost certainly covers this too.

Shifu, I'm no expect on this either. I just Googled "how gps measures speed" though and a number of helpful results appeared. In short they measure Doppler shift and use this to supplement the change in position over time, it would seem.