Well, I'm not sure I agree with the poster right above. The first part of the handspring is longer, but the second part is short. You want to snap down hard with your feet in front of you (or under you, depending on what skill comes next). I always say, "long reach, quick snap." Keep your arms by your ears as you snap down. Also, you shouldn't be sitting low into your handspring.
If your back handspring is slow, you need to do more conditioning and drills. Do box jumps, squats, lunges-- lots and lots of leg conditioning. Spend just as much time working handstand blocks and handstand snap downs and v-ups, too. On your handstand blocks, snap down in a nice, tight hollow with your arms by your ears. You should rebound backwards out of your snap down if you're doing it correctly.
Sounds like you need to be stronger and just get a little more practice under your belt. Good luck!