I currently run NZBGet in an Ubuntu 16.04 virtual machine under ESXi. I also have a FewwNAS vm under this same server which hosts all my media. I have been using SabNZBD up until recently when I upgraded to a gigabit connection and wanted to try to saturate it.
If i have NZBGet download to the vm storage, I get ~80MB/s but downloading into the NFS mount from FreeNAS i get 1-9MB/s. I had SabNZBD downloading into the same directory and it was doing 30-50MB/s. I checked the network speed with ifconfig and get 4gbps (local connection between VMs) and testing NFS with dd gets me ~230MB/s so I do not think networking speed is the problem.
I am at a loss here as to why NZBGet is so slow whereas SabNZBD works well.