[Constant01]

Never use the Gen 5 slot because it will use lanes from your PCIEX16 slot regardless if your M2 is GEN 4. To simplify it just use the M2 slots marked with a green check mark. I have the same exact board with a kfa2 (Galax) RTX 4090 and 0 issues.

 

[Constant01]

Well "never" I wouldn't go that far. The problem is accessibility especially once you add water blocks. The M.2_1(SOCKET3) and DIMM.2 locations are easy access for upgrades and or failures without draining a loop. It's a reasonable assumption and a poorly integrated once not to mention poorly documented.

Yea I get what you're saying, I would say poor implementation too. Even when GEN 5 drives are available would it be worth to to sacrifice the GPU lanes for minimal gains? Not gonna start with GEN 5 GPU's because that would render than slot void. Oh well.

 

[Constant01]

How much performance is lost on a 4090 when running PCIe 4.0 x8?

For current Gen (RTX 4090) the loss is minimal, maybe max 2-3% (still thats alot for extreme OC'ers). But since PCIe 5.0 doubles the bandwidth of PCIe 4.0 the loss could be double in the future? Cant tell for sure.