Decode speed is generally memory bandwidth bound. Prefill is typically arithmetic bound. This is the reason for mixed batches (both decode and prefill) - it let's you saturate both memory and arithmetic.

Chunked prefill is for minimizing latency for decode entries in the same batch. It's not needed if you have only one request - in that case it's the fastest to just prefill in one chunk.

I'm pretty sure the sibling comment is right about different length limits - it's because of training and model talking nonsense if you let too long.