Unloading a completely filled up storage Minecart without having to stop it sounds crazy, but it’s possible! I actually wanted to simplify and shrink down my previously posted Quadruple Item Unloader when I encountered this interesting concept of a Instant Unloader. Stacking up Minecarts is already a common thing to do, especially stacking up TNT Minecarts to get a greater explosion or Hopper Minecarts to pick up a lot of items from the ground at once, but to suck out items from a storage Minecart on top was a bit more tricky to do, since you somehow need to place a rail on top for it to become useful.
Since this is almost impossible to do in Survival Minecraft and would lag your game a lot, this is only a Proof of Concept. After some testing I found you can suck out half a storage Minecart with about 580 hopper Minecarts and if you send the Minecart back they suck out the rest, of course if you double the amount of hopper Minecarts to about 1160 the storage Minecart can be emptied just by driving by at pretty much full speed. You can easily see on your entity count on your F3 screen how many Minecarts you’ve already placed. And this is the first big problem, Minecarts are entities and the more entities you have in your world the more lag you will get. The second problem is that there is no way at the moment to put items from a hopper Minecart directly into a container like a chest or dispenser, therefore we have to suck out the items one by one with a hopper from underneath and put them in a chest which will take a long time.
After figuring this out I searched on youtube for something similar and found a video from kingofdafunk so you can check out his Super Hopper if you want some more information on this.