couple of other things,
the software iSCSI initiator will
affect your CPU and performance.
If you think about what is happening, the SCSI information is being wrapped up into a TCP/IP packet and each time a bit of data is transferred from NAS to your machine or vice versa it is being unwrapped or wrapped.
is there any specific reason for using iSCSI in your setup ?
I only ask as any speed benefits you may be gaining by presenting volumes in this method could very well be costing more to your environment in used CPU cycles.
modern NAS boxes are pretty cool in they give you these features like fileserver, SMB, FTP...you could achieve what you need to achieve using these features.
Also something else that springs to mind, as you said other machines in the network have similar problems, have you checked the health of your drives in the NAS ?
failing all of those your virus/malware thoughts are valid as well.
if you don't have an up to date enterprise AV/Malware you could use the many online AV/Malware scanners. I used to use Kaspersky online, but it's been offline for a month or so, I have also used Free Online Scanner - Online Virus Scanner | F-Secure
you could just run it on your machine to start to see whether anything untoward is happening. and then blitz the whole network if you find something....