I believe NMEA position data is broadcast once per second, but that may be configurable depending upon the receiver. I'm guessing that OakCalc does some sort of interpolation. The most accurate measurement is almost always directly off the electrical speedometer signal. It is directly proportional to distance, except in the case of wheelspin. There are some very accurate acceleromters out there, and it would be the most universal way of calculating performance as it does not rely on having an electrical speedo signal, OBD-II, or a GPS receiver.

Cheers,
Phil