Did you port forward the correct ports? Without opening the ports, it will be slow. The upnp auto port opening , might not work depending on your torrent client. I use emule for downloads and I have to port forward two ports. With the ports forwarded manually I can achieve 1200kbps. Without I can only achieve 100-200kbs when using the upnp port forwarding. It is a software client issue with the upnp port opening, not a wd issue. Check your ports!
ps if you do not port foward, it might use port 80, normal internet traffic. If isp sees torrents on this port it might tnrottle you. My isp has done this before, this is why I select weird port numbers and forward.