Loaders (and probably belts) at more than 120 item/s never worked properly but as of 0.18 they are just fundamentally broken. The game engine doesn't support it currently. See https://forums.factorio.com/viewtopic.php?f=28&t=81447
The whole mod is about compressed belt throughput, what you want is technically called double dipping.
Also, trains.
Possibly the mod Miniloaders supports belts speeds that bonkers, use that instead.
