The max speed of the axis might be acceleration limited.  For example:

SPEED = 800

ACCEL = 1200

DECEL = 1200

Say the vector length = 600 - 500 = 100.  ie the MOVE length is 100.

From the standard equation of motion:

v^2 = u^2 + 2as

u is 0

v = SQR(2 * ACCEL * length)

v = SQR(2 * 1200 * 100)

v = 490

So it will not go to the 800 set speed.