I noticed this while trying to make a compact auto furnace with minecarts. At first I thought it was a one time bug but after reloading it still seems to cause issue. Included is a video of it happening also showing the speed compared to a hopper
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Also, try using a minecraft with chest and see if that helps...

Thank you for your report!
However, this issue is a Duplicate of MCPE-57637.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
Although this report is older, the newer report is more focused and specific in its description of the behavior of hopper minecarts pulling from other hopper minecarts diagonally above.
If you still feel there is a problem with the speed of hopper minecart transfer, I suggest you pursue that through the feedback site, or create a new report detailing how the behavior you observe differs from what you expect.
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What's wrong with faster? Hoppers in Minecarts have a different speed than regular hoppers (like you explained). Right now the regular hopper speed is approximately 153 items per minute (max of 2 minutes, 5.49 seconds using full hopper). Imagine if the normal hoppers were sped up to 240 items per minute. (max of 1 minute, 20 seconds using a full hopper) That would mean 4 items per second, which would mean very compact 50 second long hopper timer (since this is not possible in minecraft right now, if you like this idea you should definitely suggest this on the feedback site). Also, how are you transferring a item from hopper minecart to hopper minecart? Whenever I do it, the items just stop... (never mind)
Note of Reference: Greater speed = Less Loading Time...