The instantaneous ground speed is reported by the GPS unit on board and transmitted via ADS-B along with the latitude and longitude.
Looking at the latitude/longitude points, I see where you see a discrepancy:
Time Distance NM AvgSpeed Calculated Avg spd from instant spds
09:30:08
09:30:23 0.4985 119.64 (145.4 + 129.1)/2=137.25
09:30:38 0.5524 132.6 (117.2 + 129.1)/2=123.15
09:30:53 0.4318 103.6 (86.2 + 117.2) / 2 = 101.7
I would have to assume that the speed is showing a lag that is not apparent in the lat/lon points, or the other way around. Who knows...?