As Paul mentioned, there are many factors that affect forgiveness. Brace height is a big one, though. The longer the power stroke of the cam (shorter BH) the longer the arrow is on the string, giving us more time to "mess up," if you will. But that's where the speed comes from.

Bow companies are making bows to be both fast and forgiving now. GENERALLY speaking, though, a short ATA bow is less forgiving than a long, and short BH is less forgiving than a long BH, and a slower arrow is more forgiving than a fast, speaking in general terms. Cams also affect it, a very aggressive cam is less forgiving than a less aggressive cam...