Mostly a mee, too (still happens with the newest version of factorio). The funny thing is it doesn't always happen, i.e. I cna have a belt wiht lots of turns and it works for the first few splitters, but ultimatey, it will slow down.
I don't think it has anything to do with compression though: The sympton is that a single splitter with a single input and output lane sometimes gets full throughput, sometimes one lane is roughly half speed, sometimes both lanes are half speed. When that happens, items will queue up at the input side, so should be fully compressed at that point. This slow-down mode changes dynamically, so something must trigger it.
Having said that, this very much looks like a limitation in factorio. Maybe the ultimate belt category just doesn't work and should be tuned down.
