I spent many hours with DLink technical support but they were never able to reach a conclusion as to what was causing the issue.

I got fed up with the slow speeds and one day removed the router from the equation and connected my modem directly to my computer via ethernet and low and behold my single file transfer speeds were now able to saturate the ISP connection. My computer is a wired connection to the router. I have checked router settings, enabled and disabled QoS, made sure UPnP was enabled and at one point even configured manual port forwarding in the router to no avail.

I can keeps stacking multiple simultaneous downloads until my connection is saturated, then the download speeds will be limited to the maximum bandwidth available from my ISP, however no download is able to exceed 2.0MB/s. Each additional simultaneous download is also capped at 2.0MB/s. I have a Phenom II X6 1100T processor with 16GB of RAM. I ensured I had no speed limits enabled and was transferring the files to a SSD. This is far from what I should be able to receive a file at.

I could connect to the server using FTP over TLS and could max out my internet connection of 100Mbps if I allowed 10 simultaneous downloads but if I allowed say only 1 simultaneous download, the download would peak at only 2.0MB/s. I too had a similar issue as described above (and am currently having the same issue on my parents computer).

Filezilla android transfer speed