Do you have IGMP Snooping enabled on your switch(es) ? This is a requirement for Multicast to work correctly.
What are you using as your Ghost server and what bandwidth is available on the NIC? Are we talking 1Gbps or 100Mbps ? It looks like your server NIC may be running pretty much at saturation with one machine so doubling the loads results in doubling the time taken.
Also, how big is your image?