Just going to add in here that when I want to see how busy my HDD’s are during tasks, I use nmon. Can run as normal user and press d to view disk activity percentage. Very friendly to a newb. Press n to also view network stats and c to view CPU as well.
If I was you, i’d bring a computer over to the same physical switch that your FOG server takes the image on. See if it sustains a better speed or not to determine if network routing is to blame or not.