I've used a flat back fall (thunderfall) on concrete at over 30 mph without a scratch or a bruise (front brake worked, rear brake didn't). I've taken several memorable falls while climbing. One was only twelve feet up, but my foot caught on a branch so it was face first over fist-sized river rocks. A judo front fall left me with fore-arm bruises and nothing else. So, obviously, I'm happy with the slaps.
The physics are pretty simple, for our purposes. Once you are in the air there is only so much energy in the system. There is nothing you can do to increase or decrease that energy, you can only focus it or spread it out, either in space (full body contact vs landing on one wrist) or time (flat fall vs roll). The palms of your hands can take a huge amount of damage for their surface area and so the slap draws damage away from the rest of the body (because the energy in the system is fixed, anything that hits harder lessens it elsewhere.)
Another note- there are some styles that teach only rolling falls. I was taught that both were important. Some opponents will let go of you and some will hang on and try to finish- if they hang on it really messes up a roll-out.
I also haven't noticed the slap slowing me down in protecting. Do you use the rebound energy?