Since there is no time component, you are looking for the magnitude of the vector. You can get it as follows:
M = Math.Sqrt(dx * dx + dy * dy)
"dx" stands for "delta x", or the x component of the vector. "dy" is "delta y". You can compute dx given any two x points:
dx = x1 - x2
Same goes for dy. If you really want a speed (not a velocity, as velocity is really just speed and direction), you divide the "M" (magnitude) by the amount of time it took to travel that distance.