Not icing is believable. The claims today say that, while icing helps with swelling and inflammation, it also slows down the recovery process because it slows down blood flow.
Now, the part about practicing every day for 6 hours is questionable although with Dice-K coming to America people are starting to think that maybe pitchers should be throwing - and even pitching - a whole lot more than they do. But the jury is really still out on that one.
Let's get one thing straight here. Do you just feel like you're throwing slower of are you actually throwing slower? I could see putting the mechanics and timing together with the slope of the mound and the effect of gravity such that you end up using your body to throw more than on flat ground. And that can make you feel like you're not throwing hard even though you are. I think pitchers somtimes associate the feeling of throwing their arm out of their socket with throwing hard.
Many people think that tightening up slows you down. I have no proof that is the case but I also have no reason to doubt it. I do think there is something to be said for a pitcher who looks smooth and effortless and I don't think you look that way when you tense up.