imu wants to get the business down quickly. the situation in the holy land is urgent. also it would make no sense to show us all of imu's abilities already. something needs to be left for the real fight, where luffy defeats him.
getting business done quickly =/= faster arc
May i remind you that Dressrosa was 1 day?
Or maybe you should look up Namekian minutes
Time in manga, especially in fights, does not flow fast at all.