Regardless of length, the amount of energy available versus the weight of the moving limb material that the available energy needs to move will greatly influence bow speeds. I have very trim and slim longbows that will blow the pants off some that have heavy tip segments. A longbow that does more of its bending near the riser will have more limb to move than one that does more bending out to the tips. With some designs the amount of working limb is much shorter which can contribute to more efficient stored energy, as example some static recurves are pretty quick, but only a few inches of the limb is compressing and stretching to develop the power. However it is compared, one must still compare the identical bows at different lengths, if the bowyer makes adjustment with longer length limbs, it is no longer apples to apples. I find that highly efficient longer bows are all around better most of the time, but there are always exceptions.