Hey Sorata4, thanks for letting us know.
Yea I also noticed the manual failed to mention the shared bandwidth with the M.2_1 & the top x16 slot. So it doesn't matter whether the M.2_1 slot is using a Gen 4 or Gen 5 M.2, the main top x16 slot drops to x8??
I know for my Z690-E the Gen speed of the M.2 using the M.2_1 slot doesn't even matter, as long as the M.2_1 slot is occupied regardless of the Gen speed being used then the main top x16 slot drops to x8, so it looks like this is still the case as you're saying.
What about the CPU's M.2_2 slot?? That's fine to use as Gen 4 & the top x16 slot will stay running at x16 right?
Sorry for late reply, these forums have been ugly lately, couldn't even scroll up/down pages until I realized was ASUS website cookie settings, geeze.
Well if you still need the answer or in case it might help anyone else, quick refresher.
ANY Gen/speed M.2 drive occupying the CPU's "M.2_1" slot. whether it's Gen 3, Gen 4 or Gen 5 will result in the main/top x16 slot to run at x8 & the 2nd/bottom x16 slot gets disabled.
If someone wanted to use BOTH the x16 slots & NOT using the CPU's "M.2_1" slot then both x16 slots run at x8.
Even 4090 graphics cards are PCI-E Gen 4, so if I used "M.2_1" then my 4090 would run at x8 Gen 4 & NOT x8 Gen 5 even though the x16 slot itself is Gen 5.0. In saying that I've read in many places that the difference in performance of a graphics card running at x16 Gen 4 vs x8 Gen 4 is MAX 5% difference, usually lower even. This is for gaming though, in other applications, like Deep/Machine Learning, AI apps etc & heavy calculations & rendering etc I'm not sure.
Personally I just never use the CPU's "M.2_1" slot so I can just leave my 4090 running at the full x16 bandwidth as I don't only use it for gaming, & I use the CPU's "M.2_2" Gen 4 slot for my main Windows Gen 4 M.2 SSD, the rest of my M.2 SSD's go into the other slots & everyone's happy.