The bug

Basically the opposite of MC-65029 (which was resolved as "Works As Intended"), see also its code analysis

Hopper minecarts transfer items with a speed of 1 item per tick, except when the hopper minecart is exactly at (0, 0, 0). In that case items are transferred slower.

Reproduction steps

1. Place a lower trapdoor at (0, 0, 0) with a chest above:
   
2. Summon a hopper minecart on top of the trapdoor
3. Put items in the chest and observe the speed at which they are transferred
   The items are transferred way slower than usual