The item transfer speed of a hopper minecart gets slower when new, empty hopper minecarts are placed within the item pickup area of the minecart. 

To reproduce:

• First, build the item distribution setup from picture 1:
  • Place 2 fences next to each other with rails pointing to them.
  • Place four hopper minecarts on the rails and push them tightly against the fences. Make sure the result looks like picture 1.
  • Now, remove the fences and use pistons to replace them with blocks. This can be easily done by placing blocks on top of the rails, as in picture 2.
  • Remove the pistons and place a minecart on the two middle blocks, such that it overlaps with the two blocks. Do not place rail on the middle blocks to prevent them from connecting with the lower rails. Make sure the result looks like picture 3.
• Now, place a stack of items in the top minecart.
  • The items get split over the four lower minecarts, as each minecart contains 16 items.
• Now, place a minecart (not on a rail) on one or more of the upper blocks and place again a stack of items in the middle minecart.
  • If needed, confirm that the new hopper minecart is in the lower hopper minecart's item pickup zone by placing an item into the top minecart and notice that it gets removed.
  • The items do not get equally split: every minecart with a minecart above it gets approximately only half of the amount of items as a minecart without a minecart above it.

This means that the items with a hopper minecart above it have a halved item transfer speed than items without, because they had only got a chance to pick up half the items as other minecarts. The speed doesn't seem to be constantly halved, as the distribution sometimes varies between different experiments with the same setup. It seems rather that the speed is on average halved, but varies over time. When placing a hopper minecart above all the four minecarts, we can see that the items get approximately distributed, but not equally, which proves this.

A possible explanation for this bug might be that the hopper minecart looks for an item in one hopper minecart every tick, but doesn't look in other minecarts at the same tick when it doesn't find an item. Since approximately half of the item look-ups will be in the empty minecart, this makes approximately half of the look-ups wasted and thus halving the item transfer speed. It also explains the variations in item transfer speed, since the minecart might be looking in the empty minecart more or less than half of the times at a given moment.

Since the videos and exported chunk (using MCA selector) are too big, I have uploaded them on Dropbox. Here's the link: https://www.dropbox.com/scl/fo/iie0tfh9cdaa6bwxi6hxv/AJ8tcAkEDEjDlmqvNrYwtBs?rlkey=nj17x9trmbvl3capdxmbda1wb&st=bxia389v&dl=0