Hi there,
I would suggest you look at three areas.
First, make sure that your storage is configured properly. Being iSCSI, make sure that you have configured the QNAP (I'm guessing it supports MPIO over two or more adapters?) to support some form of load balancing or LACP. Also how is the RAID configured on the unit and what type of drives? If you had only a small number of SATA drives for example you would probably hit performance issues when multiple users were accessing the disk.
Secondly, check your network all the way from the NAS through to the vmkernel port configuration. There are plenty of good guides out there for configuring iSCSI in vSphere. Jumbo frames aren't critical and are a hot topic for debate particularly when using 1GbE networking. Flow control is certainly something that should be enabled if supported by the switches.
Lastly, have a look at the VM performance charts you could be suffering performance degradation if the server is struggling for CPU resource. How many users are accessing the server? What is the host configuration and how many other VM's are running on it? You might find that a single vCPU just isn't enough.
Make sure you become familiar with using ESXTOP from the cli, this is a great tool for looking for performance bottlenecks and there is plenty of doco available on using it.
I hope this helps!
Cheers,
Ben Loveday