I’ve just stumbled upon this reddit post that says:
"Boards like the X570 Hero Crosshair VIII have the 2nd M2 slot connected to the chipset. Although this slot is supposedly X4 Gen4 Pcie it bottlenecks new, fast SSDs like the 980 Pro and sn850. In the primary M2 slot you may get 7000/5000 MB sec read/write but in the 2nd M2 you may get 6500/3200, way below the x4 Gen4 pcie spec.
Asus support confirmed in my support case that there may be significant latency induced when using the 2nd M2 due to the chipset and this causes lower speed. I was surprised to hear this."
Source: https://www.reddit.com/r/ASUS/comments/kaqnec/beware_of_asus_x570_m2_bottleneck_before_buying/
