This is my experience with sorting on iOS, MCPE 15.2. It seem to work better under Android.

Yeah, hoppers are a little buggy. I've tried a few things I've picked up around here. Sorry I don't have references to the users.

The big one is loop the hoppers around on themselves so the items have more than one time to get picked up. You'll also need to add away to power the hopper between the transport loop and the junk hopper or almost all the items will end up there. Un-power that hopper to drain the junk once the items sort.

I also use a comparator to monitor the first hopper in the transport loop after the input chest. It powers a hopper under the input chest. This slows down the input and helps the transport hoppers not get bogged down in items. I also built the system wider, with air blocks (except the hopper chain) between every filter circuit.

Even with both of these in place, the system I built (16 items) will get slower and slower until MCPE crashes if I'm sorting lots of items. One way to keep this from happening is to exit the game and restart if it starts to bog down. This seems to get things moving again.