I have a setup that tests how many items an inserter can move in a minute which uses matter sources. I've noticed that inserters facing south will move more or less items when the belt that they are taking items from goes left to right versus right to left. I was almost ready to make a bug report in vanilla but I decided to recheck my results.
An inserter moving items from a belt (belt direction orthogonal to inserter direction) into a chest moves 553 items in a minute in my setup, unless that inserter is facing towards south direction (grabbing from north) and the belt is right to left direction, in which case it will move 624 items. Here's an image illustrating this.
Note there is a known bug which leads to inserters being faster if they move items across chunk boundaries, so in my experiments I stayed away from chunk boundaries.
Here are the setups.