Note that these buffers are not only relevant for unread data of the user space application, they also reflect the TCP window size - this means the sending side

Note that these buffers are not only relevant for unread data of the user space application, they also reflect the TCP window size - this means the sending side knows how much room the receiver has at all times , and can throw that much data at it without receiving a ACK - so there's a fine dance between network speed , latency, and this buffer size.