What version of FOG are you using? I remember a while ago the FOS Linux engine would stall out during the partclone process then take off again.
As for the speed, I’ve heard about hyper-v being slow before, but not by first hand experience.
In can say in our environment we use vSphere and capture from a vsphere vm to a vshpere FOG server is about 6GB/m and deployment is in the area of 13GB/min. I can push out a 25GB (fat) golden image in about 2 minutes. I’m only saying this to show what is possible.
So what would be interesting to know in your setup is the hyper-v FOG server slow or is it the target computer (vm)? I can say during a fog image deployment cycle the target computer does almost all of the work in imaging, the FOG server only manages the process. In my home lab I have a fog server running on a raspberry PI3 and I get 4.5 to 5.0GB/min transfer rates so the point is the fog server isn’t a major factor in imaging speed its the target computer.