You are asking an unquantifiable, theoretical question. What level of death and destruction is acceptable under the Constitution without action being taken by the Federal government?
Unquantifiable, yes. Theoretical, absolutely not. You're halfway there. We have rules for a reason, and one of those rules is that the states have defined authority over natural disasters, and surrender of that authority is given only at the permission of the state in crisis. Turn it around, say the Feds can claim permission to rescind state authority for a level of disaster that is perhaps MUCH less severe that the current events, how would you feel then?
The British used to do this all the time up New England way. Then again, they defined crisis "creatively"...
Yes, federal response has been slower than desired, but a great deal of that rests upon the shoulders of the governor of LA, and Mayor Nagin. It has much less to do with any callousness or sloth or part of the Feds. We stood to lose a lot more as a country by throwing away a giant chunk of the Constitution just because the Feds were impatient.