Express Miniloaders load into chests at speed of 30/s while unloading at the expected 45/s. Tested with normal steel chests in case the warehousing mods I use were the issue.
Edit: Used a belt counter for correct measurements :D

I'm only able to reproduce this by causing a low power situation. For example the following setup looping between two boxes should empty out the top box if loading is slower and empty the bottom if unloading is slower. But so long as power is sufficient they stay balanced, so are both handling a fully saturated belt. When power is low though it does get unbalanced.

full power: https://postimg.cc/vcnvvtH4
low power*: https://postimg.cc/qzGN0N1m

*Fairly extreme low power example here ~20% power satisfaction.

Well I found the issue just now, though no idea how it actually works.
The mod https://mods.factorio.com/mod/zzzzStopChasingBeltItems apparently changes the way the inserters inside the loaders behave and disabling it fixed the issue.