Per se, a mounted folder is not a problem for QlikView Server. What may be affecting your performance and services is how good the connection to the NAS is and how big the files it has to move or copy from the NAS to the QlikView Server, that is where the in memory copy will be.
Make sure your NAS works with a Windows operating system and not Linux or any other OS.
Does that folder have the same configuration, security, users... and so than the Root Folder? What are the logs saying when the distribution task runs? Note that there are some NAS devices that might not work, and in any case, you need to access them as shared folders from a Windows Server \\WINSERVER\NASFolder and not directly to the NAS, even if the device allows you to set up UNC paths as the above.