I’m experiencing a similar issue to what’s posted here, but the suggested solution does not seem to be helping.
When everything is set up properly with my desktop (Lenovo Ideacentre K430), the system is unusable. Mouse movement is so unresponsive that I literally (and I do mean literally) can’t do anything with the system. However, when I connected the HDMI cable from my monitor directly into my video card on my desktop, bypassing the UD-3900 it is working beautifully. The mouse and keyboard are responsive, the second monitor (plugged into the UD-3900) displays correctly and acts as an extended desktop.
Prior to bypassing with the HDMI, I hooked it up to a laptop and it worked perfectly without any bypassing necessary. While this workaround is helping for the time being and does help me to use the 2nd monitor, the main purpose I bought the unit for was to be able to hook it up to my work laptop with a single cable. Having to unhook the HDMI from the desktop and plug it into the UD-3900, and then plugging the UD-3900 into my laptops USB 3.0 port is doable, but it’s an extra step/complication from what I was expecting.