Imaging speed is determined on Stream->Decompress->Write to disk. (Technically the speed you see in partclone is purely the speed at which it’s writing to the disk.) However, this is limited to the amount of bandwidth in use to send the data.
As @george1421 showed, from a purely networking standpoint, the maximum would be 7.25 GB/min.
This is likely where you’re seeing a variance in the Unicast vs. Multicast.
I’m assuming in the Unicast instance, you’re sending the image to a single machine. So it’s got the full GB backbone available for network streaming. In the case of multicast, it floods your entire network (subnet basically) with the packets. It also has to keep a “steady” stream so the slowest machine in the group will be the entire limiting factor for all machines.
I’ll bet if you Unicast to 5 machines and Multicast to the same 5 machines, you’ll see a semi-evening out of the speed rates.