proxy.py: [Core] Default send buffer size must be configurable

I notice that queueing a huge response in a plugins "handle_client_request()" method is very slow.

def handle_client_request(self, request: HttpParser) -> Optional[HttpParser]:
    if some condition:
        return handle_request_locally()
    # else access remote resource
    return request

def handle_request_locally(self) -> None:
    f = open('my_file', 'rb')
    file_data = f.read()
    f.close()
    self.client.queue(
        okResponse(
            file_data,
            {b'Content-Type': b'application/octet-stream'},
            conn_close=True,
    ))

Connecting to the proxy via a browser and thereby accessing a huge file results in very slow handling of the data. My question: is there a bottleneck somewhere in proxy.py or is it simply the wrong approach? Even when running proxy.py on localhost and no network transfer is involved the browser indicates that it will take hours or days to transfer the data.