I think that itachis amaturasu burns faster simply because he was able to burn thru jmans toad stomach technique in an instant and it was fast enough to burn sasukes fire itself will it was being blown direct at him.. (That means it burns allot faster than normal fire). Sasukes amaturasu burns very slow for the fact that Karin didn't burn to death and the eight tails took a while to burn down. but come on, he was able to burn through a wall of toad stomach instantly with brick wall behind it. Sasulke on the other hand takes a while .