[SOLVED] Help - if you own a ROG STRIX b450 f gaming mobo

Does that cause a problem or slow performance?
Depends which M.2 you use. The first M.2 is used it removes the SATA 5/6 support. The other slot changes the PCIE slot from x16 to x8 however that makes little difference in gaming performance (2-3 FPS max). The manual, from what I've seen doesn't specify which M.2 is which but I'd assume the one on top is one and the one on the bottom is two.

In short either (or both) M.2's are fine to use.