After watching Linus, I did this...
After watching Linus, I did this...
I assembled an AMD 16-core 3950X rig, equipped with ample RAM, a 1100W EVGA power supply, and 2TB M.2 storage. I'm using the AsRock Gaming 4s motherboard—let me know if you need the model number later... To be honest, my gaming performance hasn't improved much compared to my old rig. I switched from a 1080Ti to an RTX2070 without any noticeable gain, then moved to an RTX3070 with minimal improvement. All games run smoothly and perform well. However, I noticed the FPS isn’t as high as others claim. I came across Linus’ experience with GPU Z, downloaded it, and used it. It showed that my main PCIx16 slot was only running at half speed after BIOS changes, regardless of which slot I swapped. Then I discovered my M.2 drive was placed next to the primary GPU slot, so changing its position didn’t help. Eventually, I realized the M.2 slot uses PCIe lanes, which explains the performance drop. The ASRock manual didn’t mention this issue. If I had used two M.2 drives in RAID and had two GPUs, things would have been much better. I probably should have chosen a better motherboard.
It seems the motherboard likely includes a PCIe slot for a non-volatile memory device, such as a NVMe drive, which connects straight to the CPU. With a 3950x chipset, it only provides 16 PCIe lanes directly from the CPU. In contrast, Ryzen 5000 series processors support both 1x16 and 1x4 configurations simultaneously, which may explain why the slot prioritizes direct CPU connections. Modern motherboards now offer more NVMe ports, some using chipset pathways while others are directly wired to the CPU.
The 3950X, along with all non-G 3000 series CPUs, do have 24 total lanes of PCIe, with 4 going to the chipset, giving you access to 20 lanes for devices connected directly to the CPU. This is identical to the 5000 series. It is possible that the motherboard does not have the ability to allocate those lanes, as 1000 and 2000 series parts only had 16 lanes free, so some mobo manufacturers for 300 and 400 series boards didn't set it up for 20 lanes from the CPU. My guess would be that the OP has a B450 or X470 board that was designed back in the 2000 series days.
Then I don't know what the issue would be. Maybe there is a bug in the version of the BIOS you're using? Either way, either slot will be about the same speed, so if this configuration works, then at least you've fixed the problem.