Bad read and write speed inside VM with NFS

Hi all
Im trying to build a virtual environment with Proxmox and NFS

I have NFS shared from a ubuntu machine running 8 2TB drives in raid 6 with btrfs.
(Bonnie gives me 620MB/s write speed and 304 MB/s locally on the storage machine which I am happy with)

Checking the write speed with dd on Proxmox I get what I expect from using Gigabit ethernet.dd
if=/dev/zero of=/mnt/pve/NFS_0/output.img bs=8k count=512k
524288+0 records in
524288+0 records out
4294967296 bytes (4.3 GB) copied, 42.0547 s, 102 MB/s
As I watch the network trough put with "bmon" on the storage server I can see it starting transferring data immediately.
I get these results with consistency.
As per above result I assume the NFS mount and share is OK.

However, inside the VM's I get very strange and inconsistent write speed.
When I issue the above command and watch bmon on the storage machine it can take up to a minute before any data loads the interface.
Further the write is not stable, speed vary between 20MB/s to 90/MB/s. It also stops all together to resume 30s later... (Very strange)
The above command will report speeds varying between 7 MB/s to 55 MB/s
I have tried all different write cache modes. All will give me the same result in varying degrees.
None of the gives any consistency.
I have also tried virtio, raw, cow, -> same thing
I tested to install a VM on the local drive on the Proxmox machine. dd gives me consistent results.

The problem is only evident when running on NFS.
I'm running 3.1-28.

What can I check to figure this out?

BR
Nashway