transfer rate depends on alot of thing... bandwidth of network, device itself, your com lan port speed... etc
for average, most router and lan port go about 100Mbps, that would equate to a transfer speed of somewhere around 10MB/s if the bandwidth ain't used... if your router is gigabit enabled, then it shouldn't be much of a problem...
NAS is not really recommanded if you are using wireless, as the transfer rate bottleneck of wireless is :sweat:.