I can attest to it lovely hurting if you chop somones kick (dont do it)
Those of us who have trained and therefore know how to do this correctly can attest that it's no worse than a closed hand strike. That's the thing with doing stuff without training. You do them wrong.
Second, i belive for most people open hands move faster than closed, if not everyone.
Whatever you may personally "belive [sic]", unfounded in any actual training, experience, or education, there is absolutely no physiologic reason why this would be true.