friend. . is it really ? Are you changing anchor points ?
The length of the arrow matters, but if you are using the same arrow, that doesn't matter. I believe what Orion said about the handle design is most of it, with an arrow speed contributor.
Those are two completely different designs and the pivot point is different. Try shooting the longbow first with a high wrist and then with a low wrist, palming type hold and see the difference right there, same bow, same arrow, different grip.
Learn what your bow does and use it as a tool. We sometimes worry too much about little things.
ChuckC