There are a lot of factors that determine arrow speed. Two bows pulling the same LBS draw can be totally at opposite ends of the speed spectrum.
Dropping arrow weight is one way to increase speed, string material(if bow is safe to do so), release style or even aid(unethical for a true trad archer
) But it could be that the gap in speed is just not possible to close, The design and material of one just may be faster. Example I have 2 white oak bows both unbacked, both pulling 43lbs at 28", both have B50 Flemish strings. One is a flat bow and the other is a pyramid style bow, and both have the same working area of limbs, made from the same piece of wood. The design of the pyramid bow with its thin limbs in the speed category is just superior. As an experiment I made a continuous loop string out of speed flight (not recommended)still with an arrow 75 to 100 grains lighter in the flatbow, the pyramid is noticeably faster.
But speed is not everything, the flat bows draw is so smooth my wife prefers it since it is at the top of her draw weight, she says it easier on her after shooting several shots; and she is much more accurate with the flat bow because of the feel.