Interesting finding following an interesting article from Hardware.fr (
http://www.hardware.fr/articles/959-5/nouvel-agesa-diminuer-latence-ram.html)
Basically the latest BIOS with the new AGESA microcode from AMD introduced a memory related option to improve RAM compatibility. This options is called "Bank Group Swap" in the Asrock bios and if you leave it on "Auto" the latest BIOS will disable it (again to improve compatibility) but it will cost you performance wise. So if like me you never had an issue running with it enable before (meaning with the previous BIOS/AGESA) you're better off enabling it manually. In my case you can see the difference in the below screenshots.
ON
OFF
I tried toggling this on my asrock b450 pro 4 board, and the difference was zilch, nothing other than normal variance, I expected some kind of difference tho so I wonder if the bios option is broken on my board/bios. I tried auto, enabled and disabled.
Given my cpu is higher clock speed than yours I would say my result is closer to your off result, depending on how your geardown is set.
With geardown enabled I get 74ns, with it disabled 71ns, the cpu clock is 4.15ghz during the test on a 2600X.
Your cache and memory throughput results are all way down on mine, the ram itself on the tests running similar to yours at 3200CL16 probably different subtimings tho.