Do you think it slows down gameplay at all or roughly is it the same as writing? Is this an acceptable means of tracking damage?
If you're simply using it to track damage (i.e. you take 3, I take 1, you heal 1, etc...) then it's just as fast as picking up a pen and writing it down... and considerably faster than using dice.
... at high level events you must use pen and paper as stated in the comprehensive rules.
I'll let Ryan speak to this point specifically, but what I can say is that UDE is officially supporting this app and I've been working closely with the OP group in the interest of making it tournament legal.