The bug

When a hopper minecart is placed below a container, it starts collecting the items with a speed of 1 item per tick, whereas hoppers (block) collect items with a speed of 1 item per 8 ticks.

Reason

The reason is probably that the TransferCooldown stays at 0 all the time, whereas for hoppers (block) it sets the value to 8 after an item was moved.