Explorer can't load folders/drives with mounted network drives.

As you see in the screenshot Explorer just sits there trying to load the folder I've requested. This happens, seemingly, randomly, at least I haven't been able to find a pattern. If I unmount all network drives the problem disappears. Sometimes it loads within a few seconds while some other time it can keep loading for several minutes preventing me from doing anything useful. As you can imagine this is very frustrating. One solution to temporarily bypass this is to cut the network connection and open the folder I need and then reconnect the network.

System:
-Windows 10 Home/Pro.
-ZyXEL NAS540 with latest firmware (I've gone through several firmware updates since purchase).
-Other devices mounted as network drives for easy access (e.g, Android phone, Shield Android TV, etc.).
-Router Netgear WNR3500Lv2 with Tomato firmware 132 by Shibby (Same as NAS, gone through several firmware upgrades/-downgrades, problem persists).

The router is Master Browser and WINS Server and these features are turned off on the NAS. ipconfig /all shows WINS Server as 192.168.1.1 (router).

I have 2 WDC NAS drives plugged into my Router and on Windows [XP and later], Linux Minu and Mac OS X computers I just Map a folder on them, usually the one named Public. I can also access them through Wi-Fi/Wireless Networking on Notebooks.

I've tried mapping subfolders instead of mapping the root folder/drive but it makes no difference.