As a general rule SPS's grow outward. As the colony increases in size the growth rate increases. It will extend outward from all points simultaneously. Therefore the larger it gets the faster it grows. Now the corals growth rate hasn't actually increased any. But as a colony it is growing faster. Some corals have more surface area or points in small colonies than others do of the same size. Now this may not seem logical, but it it. A staghorn acropora frag is simple a single point to grow from. However a monticap or an encrusting coral has growth points around it's entire edge. Thus creating faster appearing growth. Does it out grow others in the same tank and size definitely. But if it were limited to only one point the growth may be very similar to other branching corals. The world may never kbnow,.,..
However you can speed the process, or at least I believe it does, by fragging that branching coral laid on it's side flat. Instead of sticking straight up. This allows the frag to first encrust on the surface it's fragged to. Then at some point it will begin growing upwards from many points. This initial growth may seem slow as the coral is not extending very fast. But soon those points grow larger and larger, more points spring up and growth becomes accelerated...
Sometimes SPS's don't acclimate very well to aquarium life. They exist, but just don't seem to thrive. Then one day they suddenly start to kick in. In my experience acquiring aqua cultured species is best. These have adapted to tank life and are usually good growers. I have a couple wild caught colonies and they just don't seem to grow well....Time will tell. Possibly one day they will kick in.