When a hopper transfers items downwards into another hopper the items move 2 at a time down to the next hopper each time the hopper moves items. However, when a hopper transfers items across to another hopper that is next to it, it only transfers one item at a time. This causes huge issues in any sort of sorting system as only half the items that you want in the chest actually get to the chest.
In a downward hopper pipe, you get double transfer speed because the one above pushes while the one below also pulls.
This is working as intending per MCPE-59494.