When unloading to a splitter, inserter needs to wait much less for the dropoff location to free up, because every otehr item is immediately moved to the other side of the splitter. Please add calculation that accounts for this.

Came here for the same thing. Throughput is significantly higher. With stack inserters it's only slightly slower than box to box.

IIRC the item is put through the splitter in the same tick (or a few ticks after) it is placed on the splitter. Therefore it effectively places at something approaching double speed on splitters (like if the belt was twice as fast)