Because I often tend to upgrade my belts piece by piece, I came across the following observation:
When unloading from a chest, the speed seems to work correctly, in that the yellow vs red vs blue ratios reflect the ones of the belts and each tier fully saturates the belt of the respective tier.

However when loading a chest, it seems that all tiers load at (approximately) the same maximum speed. To verify, I've tried a simple setup in sandbox mode (all tech) like in the following schematic ([] is a chest, -xxx> is a loader of some tier and ... is a blue belt):

[] -blue> ... -yellow> []
[] -blue> ... -red> []
[] -blue> ... -blue> []

While the blue one seemed to finish moving the content a bit faster than the other two (which were very similar in time), this doesn't reflect the expected ratio of 1:2:3 at all in my observation.

As a workaround, I can just be "fair" in the sense of always using the correct loader for a given belt (especially for looks) and everything works as expected - but the behaviour doesn't seem correct nevertheless!

I have seen a related report for chutes (https://mods.factorio.com/mod/miniloader/discussion/5fc721dec89bef87a7babfa7). If this is actually the case for all the tiers, I suggest putting a note about it to the FAQ!