Originally Posted by Americonfusion
They are all on 1x-16x powered ribbon risers
For lack of better ideas, I'm swapping out Windows 8.1 x64 for Windows 7 x64 to see if there is any difference. I know that at least 2-3 people with 5+ GTX 750 Ti's have achieved 300+KH/s per card concurrently on Windows. I haven't been able to figure out the differences between my setup and these other people yet. I know that using Ubuntu 12.04, I do see better performance/scaling when running the cards concurrently.
I just tried two different systems running Windows 8.1 x64 with a GTX 750 Ti plugged into a x1-x16 USB3 riser assembly. Both perform consistently bad.