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