why sending data after headers and body fully get ready

[togear]

I'm wondering why in this module QUIC's transmission method is not streaming, but rather waits to download the headers and body fully before sending to the client. Intuitively, it seems this could be detrimental for fast response times, as it likely increases the first packet delay. What were the considerations for having QUIC wait for all data before sending, rather than streaming chunks incrementally?