I just tried uploading a 48Mb file to the Duet 085 in my Ormerod, and it completed at an overall rate of 718kb/sec.
Some possible causes of slow uploading:
Bad or old network switch - can you confirm that is it a 100Mbps or 1Gbps switch, not an old 10Mbps switch?
SD card performing very slowly - try another one
Other traffic being received by the network causing congestion, possibly spam from outside if your network has any ports open to the Internet.
If it is a network issue then a Wirechark trace might help to pin it down. But I suggest you try a new SD card first.
HTH David