I've done a simple test on my little home LAN.
I have enabled broadcast ICMP echo responses with
on just a single host and then pinged a multicast address to see the responses of that host. The round-trip time was consistently around 2-3 milliseconds.
--- 220.127.116.11 ping statistics --- 1000 packets transmitted, 1000 received, 0% packet loss, time 1000322ms rtt min/avg/max/mdev = 1.956/3.666/163.318/10.781 ms
However, unicast ping of the peer directly by its IP address measured 0.7-1.5 milliseconds.
--- 192.168.8.23 ping statistics --- 1000 packets transmitted, 1000 received, 0% packet loss, time 999270ms rtt min/avg/max/mdev = 0.498/2.459/227.312/12.608 ms
For the sake of completeness, the LAN has wifi link bridged with a 100Mbit Ethernet between the hosts.
So, what's the explanation? Could that be a software problem? Does broadcasting theoretically imply increase in transmission latency? How?