Folder navigation and file transfer on Apple Mac extremely slow

When accessing the ReadyNAS RN104 from a 2011 MacBook Pro through Finder (whether the app itself or from inside another app's open dialog) it takes an extremely long time to populate a folder or transfer a file. Clicking on folders in finder can take several minutes of the wait cursor spinning before the folder is sudden;y populated. It can take up to ten minutes to transfer a file of a few KB. This problem appears to be intermittent and random and sometimes will work correctly but more often than not the issue is there which makes the NAS virtually unusable. 

I have the ReadyNAS RN104 installed on my local network attached via ethernet to my wifi router. The IPv4 address is set using DHCP from the router and DNS is working correctly as I can ping the NAS unit by hostname from all systems on the LAN. Firmware on the NAS is version 6.6.0.

I have two MacBook Pros and the issue affects both of them, while I also have two Windows 10 laptops which have no problem at all. The issue occurs on the Macs whether we use AFP or SMB to access the NAS and happens whether we connect over wifi or a direct ethernet connection to the router (negating wifi problems as a factor).

I have had the RN104 for nearly a year now and this issue always seems to have been there. 

Thanks in advance