NAS as iSCSI Storage for VM

[#1]

We have had to move a Windows File server off a SAN to a NAS to increase storage and both read and write performance has dropped dramatically and impacting users. I am looking for suggestions on how to increase performance. Even just navigating through the folders in Windows explorer can be

Originally the VM with 5TB storage was on a Fibre Attached SAN to VMWare hosts. I have moved the whole VM to a Qnap TS-1273U-RP with a 10Gbe iSCSI connection to the Vm Host. The TS-1273U-RP as 8 x 10gb Disks in a Raid 6 with 1 x 250gb M.2 SSD cache.

Trying to look at all the options of how we can boost performance.
1) We have 4 free bays - Adding SSD Drives and using Qtier?
2) Adding 4 x large capacity SSD drives and using as primary storage?

Any other suggestions to get the best out of this NAS? 

Second question, does the location of the OS make a difference on performance. Eg Store with the File Disk on the NAS or store separately on the SAN. 

[degvm]

Doublecheck latest drivers for your network card, also keep an eye on the firmware. Try to find a best practice guide for your storage system, perhaps https://www.netapp.com/media/10720-tr-4067.pdf can help to find some advices

Also have a look on the advanced parameter for tuning in Esxi: https://library.netapp.com/ecmdocs/ECMLP2843689/html/GUID-346ACB95-6AD4-4DEA-8901-C9697AC3530F.html

NFS settings

Net.TcpipHeapSize = 32

Net.TcpipHeapMax = 1536

NFS.MaxVolumes = 256

NFS.MaxQueueDepth = 128

NFS.HeartbeatMaxFailures = 10

NFS.HeartbeatFrequency = 12

NFS.HeartbeatTimeout = 5

These are for NFS, perhaps try that also.

Your VM should have different vmparavirtual Scsi - Adapter for each virtual disk to parallel the IOPS whithin the maschine. Also fragmentation of the disk is an issue, keep your disks defragmented.