Hoppers cannot pick up an item stack into more than 2 slots at at a time.

How to replicate:

See attached video (sorry about the quality and audio it was recorded with a laptop camera)

1. Place down two hoppers with a one block gap between them.
2. Place a block with an inventory (e.g., dispenser, chest, hopper) underneath one of the hoppers such that items can flow from the hopper into the other block.
3. place a lever between the two hoppers and flip it in order to lock both hoppers simultaneously.
4. drop 5 stacks of items (maximum amount a hopper can hold) on top of both hoppers.
5. flip the lever to unlock both hoppers simultaneously.

Expected result
The hoppers collect the items at the same speed.

Observed result
The hopper transporting items picks them up more slowly.