How to speed up copying a file to a socket

Question

I am using tokio to copy a file (tokio::fs::File) to a socket (tokio::net::TcpStream):

let mut buf = [0; 1024];
while let Ok(n) = file.read(&mut buf[..]).await {
    if n == 0 {
        break;
    }
    socket.write(&buf[..n]).await?;
}

But I noticed that this code runs much faster:

tokio::io::copy(&mut file, &mut socket).await?;

Even when the buffer size is the same, the copy_buf call is still much faster:

tokio::io::copy_buf(&mut tokio::io::BufReader::with_capacity(1024, &mut file), &mut socket).await?;

Why? How could I optimize the first code block?