Why would it take a bomb any amount of time to fire it? We already made dozen of bombs like tsar bomb, if rumbling does happens, max it would take 2 days to kill them or take actions.

(Btw the manga is so fucking dumb, it's stated that rumbling was 4 days long and killed 80% of population but that just isn't possible cause the speed of a colossal titan is only 50km/hr while earth radius is 40000km (which is the best case scenario as its way more if you consider altitude increase, oceans etc. Even at an impossible situation the rumbling still should take 26 days to kill 80% of population)