Amaterasu is inconsistent.
One time it burns a large whole in a toad in mere seconds, another time it takes ages to burn some trees. Another time it didn't even kill Karin.
Does it burn? Yes, but not as instant or fast as people think, Itachi burned through a window, there was nothing on the other side, then again, it couldn't kill Karin. Sasuke also had enough time to use oral rebirth before burning against Itachi.
The same with Hachibi, it was struggling of pain in the water, but yet, if it burns that fast, then Hachibi would have burned completely very fast, yet it didn't.
Raikage is another example. You see Amaterasu failed in doing (didn't burn him, he had enough time to cut off his arm). It failed in doing something what Sasuke/Itachi fans say it does more than accomplishing what they say the jutsu does.
It has a slow enough burning speed so a person can counter before it burns him/her completely.