when using blue belt x16 or above, splitting the items with a x16 splitter or above doesn't get all the items through in a single frame fast enough. this leads to a hard cap of many items a splitter can split in a single frame, which I think is around 480 items/s
edit: I found a workaround, apparently it's not the speed of the splitter, it has something to do with the speed of the belts after the splitter limiting it. by using more x16 belts after a x16 splitter, then putting x8 belts after that, it works like a charm.