Filtering can have a big effect at any point where velocity is changing at a high rate, i.e. at the beginning of an acceleration run, or when the brakes are pressed at the beginning of a brake run and then again when the car stops at the end of a brake run.
On Acceleration runs
Too much filtering will cause a delay in the logged data of the point at which the vehicle starts to move, which will cause the results to be too short. On an acceleration test between two nonzero speeds, the results will not be affected by filtering.
On Brake Trigger stops
When the brakes are initially pressed and the vehicle starts to increase its level of deceleration, too much filtering will cause a delay in the velocity, which effectively holds a higher speed for longer before it shows the deceleration. This extra period of time at a higher velocity causes the resultant calculated distance to be too long.
On Brake Trigger and Brake stops
Too much filtering will cause a smoothing of the short period of time that a vehicle reaches zero before the 'Rock Back'. This can cause this point to be missed by the processing software and hence the distance and time calculated to the point after the 'Rock Back' will be too long.