I try to keep alk as stable as possible (manual dosing at the moment), in the 8-9 dKH range. I run barebottom, low nutrient tank utilizing All-In-One biopellets/skimmer, etc. Having no sand allows me to have insane amounts of flow (MP-10 shortest most intense Long Pulse Mode + WP10 in Else mode running at 75%). Lighting is DIY LED's, 28x3watts running at about 700mA over my 25 gallon cube. I only have two fish and am extremely careful when feeding them (only a couple pinches of pellets every other day and I make sure they eat every one).
All of that stuff counts for something, but the main factor in SPS growth IMO is feeding. I target feed with a pipette (with pumps off) and broadcast feed a mixture of Nutri-Cell, BRS Reef Chili, and AcroPower every other day (about 35mL tank water, one scoop Nutri-Cell, one scoop Reef Chili (both measured with the scoop that comes with Reef Chili), and 2mL AcroPower). I also use Red Sea Reef Energy A&B about every 3rd day (4mL each)
I also agree with the above statements that smaller frags take longer to take off and once they reach a certain size, they tend to grow faster. Finally, it should be said that some acros encrust much longer than others before they start shooting off branches. For that reason, I try to isolate acros with larger overhangs so that they can only encrust to a certain degree. Then, they have no choice but to shoot up a branch if they want to keep growing without encrusting into the shadows (which they still do on occasion (I HATE YOU BONSAI ACRO!!)).
Okay, enough chatter. Here's the results of my technique (which I'm continually attempting to hammer down). I made the animations by taking images once every two weeks, hense two weeks between each frame. Total growth time is 3-4 months.
Palmer's Blue (prostrata)
Random LFS acro I believe is an acropora nasuta