If you use risers that have the USB cable come straight out, clearance is not an issue at all.
Right-angle risers you have to use in the "closest to the power connection" slots if you are going to use more than one riser per row. or you get cable interference that can make reliable seating of the riser "interface board" problematic.
There should be no performance difference between the slots - try swapping your risers around to see if the issue follows the board.
I've had issues where a "performance difference" turned out to be a marginal USB cable, and just replacing the cable with a better cable CURED the issue.
I have risers that go straight out so that should not be an issue, thanks.
there should indeed be no performance difference between slots but the reality is that I do have differences. I contacted Asrock and they suggested to return the motherboard. I have tried the low performing slots with other gpu, risers, cables, ... same result. I have some well performing ones so it was easy to test the same hardware on the faulty pcie slots. Strange but now it will cost me more money waiting for a new motherboard rather than buying a new one (I make more than 50$/day with my rig so anything above 4 days waiting is wasting money)