Not to get into the Do some notice a push more than a whack (the "Get It Over With" G T Thomas school of pain thresholds)?; the pressure biz can be stated two ways. A relatively more inertial load (read more shot) "needs" a relatively slow powder to prevent pressure spiking too high before the load gets moving and to maintain pressure over the time necessary to defeat inertia. The relatively less inertial load (less shot) "needs" the faster igniting powder to avoid outrunning the burn rate over time, increasing the volume which gas must fill, and thereby decreasing pressurization (and efficient burn). The other way of saying the same is that the fast or slow powder needs its appropriate shot charge as a blowout plug in the vessel of reaction. Expanding gas is sure necessary to do the work, transfer energy, or whatever, but where is there shown to be a direct correlation between small-p pressure and velocity in Newtonian mechanics. Others besides Thomas are "hooked on a feeling".

jack