Use the for statement to call CopyN(io.Discard, r.Body, 10) in a loop and sleep after each chunk.

for {
    n, err := io.CopyN(io.Discard, r.Body, 10)
    if err == io.EOF {
        // Body successfully discarded.
        break
    } else if err != nil {
        log.Fatal(err)
    }
    time.Sleep(time.Second)
}