You should increase benchmark size as I wrote and you will see that your iscsi peak advances go away.
Edit: It's not uncommon for a pve cluster to have >100 machines (vm or lxc) and a multiple of 10TB of data and you cannot exapolate benchmark results of 8 to 16 GB based on your prefered iscsi protocol to eg 16TB in reality.
This is using 128GB test files, I knocked up everything to be WAY above the RAM available to the client and host.
Whatever Pure's file system is, which I'm sure is some proprietary erasure coded file system.What filesystem do you use under the hood of your nfs ?