XR500 slow LAN & 5GHz speeds

I have an XR500 router and I never seem to get close to 1 Gig speeds when my 1-gig-capable devices are connected to it.

I have a My Cloud Pro Series PR2100 NAS storage device with 2x4TB WD Red NAS Hard Drives in. The NAS is connected to a Netgear X4S D7800 router which is in 5GHz bridge mode:

PC <Ethernet> XR500 <5GHz> D7800 in 5GHz Bridge Mode <Ethernet> NAS

The NAS is connected via Wi-Fi bridge because of the ticking sounds from the hard disk actuator arms and therefore I do not want it in the same room as my XR500.

At first, I thought that the Wi-Fi link was the issue for slow speeds, so I plugged my NAS directly into the XR500 with a Cat 7 ethernet cable. The speeds I got transferring a 1GB file were:

605mbps download, 215mbps upload.

These speeds are nowhere near the capabilities of a 1 Gig port with a device capable of 1 Gig transfer speeds connected to it.

If I take the XR500 out of the loop and connect it directly to My PC, the speeds I get  are approaching 1 Gigabit speeds, confirming my PC & NAS are both capable: 

880mbps download, 864mbps upload.

Can someone please advise me on what might be wrong with the transfer speeds when my XR500 is in the loop?

Thank you.

P.S.
I have already raised this issue with the people over in the NETDUMA forum. If you want to brief yourself on what we have tried so far, the thread is here:
https://forum.netduma.com/topic/30244-slow-send-rate-over-5ghz-wi-fi/