Constant PARALLEL_BYTES 

const PARALLEL_BYTES: usize = _; // 1_048_576usize

Bytes of tiles past which a read copies its rows of tiles on COPY_THREADS threads (SharedFramebuffer::copy_tiles); below it, starting them costs more than it saves. Scoped threads of the copy’s own rather than the rayon pool: waking the pool’s every worker for a copy a frame kept them spinning, 1.9 cores more at 2560x1600 and 60 fps.