Stack Inserters will wait FOREVER until they can pick up a full stack of input items before dropping them on the destination.

The best effort stack inserter will pick up as many as possible, then drop the stack on the output. This requires a decider combinator to override the stack size if we are holding a less then full stack.

https://factoriobin.com/post/kuva7t

I'm using this to compact up sushibelts with great impact!