Any extra weight in the limb anywhere slows it down some. So the lighter the limb, the faster, all other things being equal. Narrowing the limb is one way to decrease limb weight. However, in most bows, unless there's a corresponding increase in limb thickness, It also lowers draw weight. There are always tradeoffs.
ACS limbs get their speed from a number of sources. One is the patented concave/convex limb cross section. Two is the carbon used on the back and belly (lighter and stiffer/stronger for its weight than fiberglass). Three is the thin core material. Because of the design and carbon back and belly, very little core wood is needed; a couple of thin maple actionwood laminations usually suffice. Bamboo is also an option and is even lighter.
Because of the limb design and carbon, ACS limbs are among the lightest and fastest longbow limbs available. Their limbs are thin, but not particularly narrow. They also use carbon for limb tip overlays, keeping weight to a minimum there as well.