Velocity showing in GPS-Navigation Systems
This depends upon a large number of variables that are imposed by the GPS navigation system in use and by the processing of the data by the SAT-NAV following the determination of the position fix.  These include


The interval between position fix solutions.  This is normally 1 second.
The position accuracy of each of the positions used to determine the velocity.  This depends upon the quality of the sky view and, to a certain extent, the length of time that the GPS has had a sky view.
The number of positions used to calculate the velocity
The acceleration of the unit 
Filtering algorithm used to smooth the velocity value 


The velocity value from the GPS receiver is calculated from the difference between position fixes.  As these are typically generated at a 1Hz rate you are measuring a distance travelled in 1 second.  The position fixes can have an error of a number of metres, with successive positions wandering from the true position, sometimes by a metre or more for a static unit.  I have had static units using high end GPS chipsets that have apparently had a velocity of up to 2 m/s while fixed to my office building.

Some units may filter the velocity value or use more than 2 position fixes to calculate the antenna velocity.  This will add a lag to the velocity value shown as the antenna accelerates.

Providing a definitive answer is not possible in just the same way that defining position accuracy has to be bounded by many caveats around the sky view and number of satellites visible.