I saw one other user had this problem, but it wasn’t resolved, as far as I could see. When I plug in my UD-3900, it reduces my internet speed significantly. I’m on an ASUS laptop UX303UA, i7 6500 processor, 2.5Ghz; 12GB RAM; Windows 10 64bit.
I’ve tested a few times, and over 6 tests, the average with UD-3900 UNplugged was 11.58Mbps; with UD-3900 connected, it was 4.97. The UNplugged scenario ranged between a top speed of 12.11Mbps to a low speed of 10.63; the connected scenario ranged from 1.45 to 8.21. So the effect of plugging in the UD-3900 was sometimes not really noticeable, but other times saw the speed absolutely crash.
I have tried: (a) plugging the unit into a different USB port on my laptop, with no real difference; (b) using a different USB cable - a non-USB3 cable, again no difference; © moving the UD-3900 as far away from my laptop as possible - which seemed to make a difference for a while, but the benefit seems to have stopped although nothing else has changed in my setup; (d) powering the UD-3900 down and back up - this seems to increase the speed but, like the other user, only for a couple of minutes before it drops again; (e) re-booting the laptop - again, a couple of minutes of improvement before dropping again; (f) checking it’s not the browser by using two different browsers - no real difference. The Wi-Fi connection isn’t experiencing interference (I work on the floor up from the router) - the range 10-12Mbps is what I get when connected by cable directly into the router.
In every other way the UD-3900 is great and does everything I want it to do, but the ‘friction’ associated with websites not loading, emails taking ages to upload, forms not working and so on means I can’t see it as a long-term option unless I can resolve this.