I'm having the same problem for over a year now. Changing to Gen3 does not help. I can switch it to Gen2 and performance is cut to half, about 400MB/s.
I noticed that it'll be x1 when the motherboard is cold. After gaming and rebooting, it'll switch to x4.
Is yours consistently booting in x4 now? Try checking the morning when the motherboard has been off and has cooled down.