I want to ask the question why do you need the feedrate? The velocity of the various slides does not give you the true feed rate. Say you have a 1" diameter mill and you are milling a .625" radius the periphery of the cutter is performing the actual feedrate while the centerline of movement should be slower to give the correct chip load in inches per revolution per tooth. Most of the time the information which is usually useful is the cycle time of the machine. If you are tyring to improve the cycle time by maximizing the feed rate and thus reduce the cycle time. What you really need to maintain is the chip load being consistent. There are times when even this does not work due to poor chip evacuation from the cutting zone, poor coolant flow, material being either on the high side of hardness specification or on the low side causing gummy material impact the machining characteristics. Also think of a 5 axis machining center rotating on the B axis and the X, Y and Z slides simply not moving. What is the velocity then, zero.
You are not seeing the whole picture with slide velocity.
Bill