QNAP poor performance

I have a QNAP TVS-671-i3-4G, whenever I am writing data to it, such as from my iMac, or NZBGet is post processing, everything slows to a grinding halt. I'll initially get the full speed of 1Gbps but this will slow down then speed up. There's no AV anywhere, and previously with a Synology 415+ everything worked perfectly.

There's nothing in the logs on any device, or on the switch. I don't think it's network related as when NZBGet is post processing, it all happens internally on the NAS.

Any ideas?

What % of the CPU is being utilised during the transfer?

Next to nothing.

Are there any benchmark apps you know of I can install on the QNAP?

If not I may do some dd if=/dev/zero > of=/tempfile with varying sizes and see if iostat is installed or something.

I had slow downs on my Qnap 8 bay. I reset my BT hub and the speed went back up.
I still have to do it now and then.

It happens internal though, so not sure how the rest of my network could affect it.