Getting closer to the bottom of this.
On AWS, problems occur once I scale beyond 16 CPUs.
c3.4xlarge (16 CPUs, 30GB memory), v.net scales linearly
c3.8xlarge (32 CPUs, 60GB memory), v.net doesn't scale AT ALL - the same as having single CPU.
I don’t believe its a pg issue, as I run loads of 32 CPU parallel scripts with postgresql as a backend
Also, I noticed the same drop off issue with 32 CPUs when I had sqlite as the db driver in grass.
For now, I can work with 16 CPUs.
Time to enlist the help of a sysadmin to find the bottleneck.