Example: When I have a full blue transport belt feeding items to a yellow miniloader, it just takes them and puts them in a chest at 45 items/sec.
The inserters probably take the items too soon, so they aren't slowed down by the slower belt.
Is this a known bug? Is it possible to fix it?

I have here something else related to speed. I have made some custom setting for beltspeed and try it for the loaders to. At first it seems it wouldn't work because the gui shows the old throughput. But when i make an test, it seems that the new speed would corespond with my custom settings but it hasn't come into the gui (description?). Is it possible that we not see the correct throughput for the loaders after other mods change it somehow in the late data-final-fixes?

That’s correct. The belt speed is put in a custom description during the data stage and will not reflect changes made in the data-final-fixes stage.

@villfuk
It is not easy to fix this because when inserters pick up items from a belt that immediately makes them disappear, opening up new items to be pushed onto the belt by a faster external belt e.g. a blue belt feeding onto a yellow miniloader. This would be possible to fix if Factorio supported “locked” inserter arms, but that is not currently possible.

So I assume you can't just push the inserter pickup spot further into the tile. Let me try...

I found a solution by limiting the inserter rotation speed. By changing the rotation speed, you can get the max loading speed to 1 item per any natural number of ticks. By using 0.5, you get one item per tick -> 60 items per second, which is the maximum speed. You can divide 0.5 by any whole number to get a lower speed. For normal loaders, I use 0.125, for fast I use 0.25 and for express, I leave it at 0.5. This means the respective max loading speeds are 15, 30 and 60 items per second.
As long as extension speed is sufficient, it does not interfere with the loading and unloading speed, so it can be used to make the entity show any max consumption you want, for example, double the actual consumption at max speed, because there are two of them.

I used to do this (have different rotation speeds per tier), but inserter timing on pickup is so unpredictable especially under lower power conditions that I abandoned it as too complex.

and inserter stack bonus messes it up too

and inserter stack bonus messes it up too

By default Miniloader inserters ignore stack bonus, but yes, if the user changes that setting limiting hand speed doesn’t work as expected anymore.