speed does matter and it matter a whole lot. deer jumping the string, trajectory, energy are some of the reasons. a certain amount of energy is required for proper penetration but that can be gotten from speed or mass, try shooting a lead pipe in your bow, see where that gets ya. true, a lighter arrow will slow down faster, same for a bullet but that is at a range you need not be overly concerned with. with lighter bows and lighter arrows use smaller broadheads like three blade or especially two blade that have been honed. over a large number of kills, the best of them are from pass through and the reasons are many, forget the expanding kind, even if they were any good, you won't have the power in a light bow to open them. my indian archery 45 lb recurve from the 70's shoots a hunting arrow, 2016, at 170 fps, that's pretty reasonable and with a set up similar to that have had a complete pass through on an elk shot though the lungs at 30 yds. any recurve that can shoot a hunting arrow at barely over 200 fps. is unusual and really fast

cheers