My homegroup transfers files at around 850Mbps, which is the limit of my gigabit Ethernet network (when you subtract out the transmission overhead) but all of my computers have SSDs that also helps the speeds. Basically the slowest component in your network will be your limit on transfer rates.
The real issue with homegroup transfer speeds is if you are using wireless on a member of the group -- for some reason it seems that there is about a 1MB/s cap on homegroup wireless transfers and many don't even get close to that rate.
I don't think that it has been fixed (AFAIK), but you can just turn off homegroups and use a standard workgroup with file sharing and avoid the problem if you are using wireless with your network.
Also, insure that your wireless has optimal settings (if possible) -- N only and WPA2 with AES encryption so that you are not limited to 54Mbps.