uhhh, maybe you need to stretch your wrist or something.
I went downstairs and smacked up my heavybag a few times palm striking....I see two problems:
First, if you keep your wrist pulled back, fingured strecthed backwards to clear then out of the way...that's a lot of muscle tension on the forearm, which felt like it was slowing me down a lot.
The threat of REALLY hurting your hand or wrist when punching is much more prevalent.
Second, keeping the hands loose until point of impact seemed to remedy the tension/speed problem, but then there is a huge HUGE risk of not getting your hand set properly on impact, if your fingures, hand, w/e isnt cleared out of the way in time or lands funny, you very well could hurt your hand and make you even more unable to defend yourself.
==Just my thoughts==