I have a PCI USB riser like this one:
I have 2 identical video cards with one plugged directly into the motherboard and one plugged in via the PCI riser. I am seeing a hash rate of 290 KH/s on the one directly connected to the motherboard and 210 KH/s on the one connected via the riser. The riser is connected to the PSU so it is powered. There is no need to short the PCI slot. I have tried this riser in both a PCI x1 slot and PCI x16 slot with the same results. The video cards are both GTX 580.
Could the added latency of going through a USB wire account for this decrease in hash rate?
Using another video card (GTX 750 Ti FTW), I was able to confirm a decrease of about 100 KH/s while connected to the PCI USB riser. I have tried 2 separate risers with 2 separate video cards and every time a PCI USB riser is involved, my hash rate is lower than when connected directly to the motherboard. The motherboard is a Gigabyte GA-X58A-UD3R. The BIOS has been updated and I only see BIOS options to change the PCI Express Frequency and PCI Express Clock Drive.
I am having a hard time finding an answer online and I need experts, please help. If these USB risers decrease the hashing power then I'm not sure why people want them. I want them for the added distance I need between my motherboard and GPUS in my setup but perhaps I should alter my setup to allow for the ribbon cable riser version. I appreciate your recommendations. Could there be a setting that accounts for this latency and keeps an optimal hashing power?