As modem speeds increased, the fixed delay needed to send the <ACK>/<NAK> grew in proportion to time needed to send the packet. For instance, at 2400 bit/s the packets took only 0.44 seconds to send, so if the <ACK>/<NAK> still took 0.2 seconds to make it back (this is "latency" in the network, not throughput), the throughput has fallen to under 60%. At 9600 bit/s it is under 30% – more time is spent waiting for the reply than is needed to send the packet.