My motherboard has three PCIe 3.0 slots: PCIEX16/X8_1, PCIEX8_2, and PCIEX4_3 and from what I know, they all have different bandwidths (the 16 has the most, 4 having the least)
I tested this out with a PCIe SSD (the Intel 750) and got about 30% better by plugging it into the top lane (x16) vs the bottom (x4). For some reason I could not plug it into the x8 so I couldn't test it. This speed increase makes sense as the top lane has the most bandwidth between the CPU.
However for my graphics card (gtx 1080) I couldn't really see any improvements depending on the lane it's plugged into in FPS or benchmarks. Granted I haven't really done a thorough testing so my results could be off. Googling this question has people saying various things too.
So the question is: Should I get better performance depending on which slot I use for the graphics card? If not, why?
52 Answers
In theory you should use the x16 slot in single card and x16 (in x8 mode) + x8 for SLI. That said, the maximum theoretical bandwidth of PCIe 3.0 is twice that of PCIe 2.0, and most cards would work fine on the latter. What you've found seems backed up by other sources too - gamersnexus found the same results in their tests, and puget systems (who build gaming rigs) found oddly inconsistant results
Practically speaking, the main advantage of the x16 lane should be geography, especially with a 3 slot card. A well designed motherboard would not block any other usable ports.
No one's ever tested PCIe/videocards on x4, and those should use the smaller connectors anyway. There's no way I see plugging a video card into a standard, non open ended slot to be possible.
2It entirely depends on the make of the slots and the physical size of the slots which equates to the lanes actually traveling to the slots as explained at hardwareSecrets.com.
Assuming that the PCI version for each of slots is the same (which optimally will match your graphics card) and that the lanes connected are not deceptive to the size of the slot - they should hold the same performance minus whatever (practically theoretical) difference there would exist from different length of the wires used.
