FastAPI's (Starlette's) `StreamingReponse` is slow when streaming file-like objects. This gist demonstrates how to quickly stream bytes as 1MB chunks back to the client.

stream_bytes_io_quickly.py

CHUNK_SIZE = 1024 * 1024 # 1MB chunk size

def read_bytes_as_chunks(bytesIO: io.BytesIO) -> Generator[bytes, None, None]:
    # Make sure we're at the start of the buffer
    bytesIO.seek(0)

    with bytesIO as b:
        while chunk := b.read(CHUNK_SIZE):
            yield chunk


@router.get("/")
async def download_byte_stream() -> StreamingResponse:
    my_bytes = BytesIO(b'lots and lots and lots of bytes!')
    filename = f"my_file.txt"

    return StreamingResponse(
        content=read_bytes_as_chunks(my_bytes),
        media_type="application/x-zip-compressed",
        headers={"Content-Disposition": f"attachment;filename={zip_filename}"},
    )