RE: IBO Speed
FPS means Feet Per Second and it's just the unit of measure. For example the bow you are mentioning most likely had an IBO speed of 310 feet per second. Now IBO speeds are measured at:
350 gr. arrow
70# draw weight
30" draw
as btomlin already mentioned. So if you go up or down in arrow weight or in draw length or in draw weight it is going to effect the actual speed that the bow will be shooting with your particular setup. Also, if you add accessories to the string it will also decrease your speed.