Constant STREAM_BUF_SIZE 

pub const STREAM_BUF_SIZE: usize = _; // 65_536usize

Read/write buffer size used for streamed response bodies and spooled uploads.

A 4 KiB buffer turns a 1 GiB transfer into ~262k reads and ~262k socket writes; 64 KiB cuts both by 16x while staying well inside cache. This is the size nginx, hyper and tower-http settle on for bulk transfer.