In certain configurations a combination of express splittrs can have reduced throughput. I verified that I can replicate this issue without mods.
As shown in the screenshot the throughput of the four belts is not 100%.
A measurement results in a throughput of about 150 items per second while I believe that it should be 160 (four express belts).
I tested if the distance between the splitters has an impact on this problem, but found the same problem with distances up to 6 tiles (did not test further apart).
It also happens on vertical belts.
Maybe this is related to viewtopic.php?f=58&t=49180
A minimal version where I noticed this issue consists of two splitters placed just next to each other on two belts.