The primary PCIe slot on your ASUS Z790 D-4 motherboard is faster than the secondary slot due to differences in the number of PCIe lanes and their connectivity to the CPU and chipset. Here are the key reasons:

1. **Direct CPU Connection**: The primary PCIe slot (usually labeled as PCIe x16_1) is typically directly connected to the CPU. This allows it to take full advantage of the maximum number of PCIe lanes and the fastest possible data transfer rates.

2. **Lane Allocation**: Even if you are using an x8 GPU card, the primary slot might be configured to support up to x16 lanes, providing more bandwidth compared to the secondary slot, which might be limited to x8 or even x4 lanes depending on the motherboard design.

3. **Chipset Connectivity**: The secondary PCIe slot is often connected to the chipset rather than directly to the CPU. This connection can introduce additional latency and reduce the available bandwidth because the data has to pass through the chipset before reaching the CPU.

4. **Bandwidth Sharing**: On many motherboards, secondary PCIe slots share bandwidth with other devices, such as M.2 SSDs or additional PCIe slots. This can further reduce the available bandwidth for the secondary PCIe slot compared to the primary slot, which usually has a dedicated connection.

To illustrate this with an example:
- **Primary PCIe Slot (PCIe x16_1)**: This slot might operate at PCIe 4.0 x16 or x8 if the GPU only uses 8 lanes, directly connected to the CPU, providing up to 32 GB/s (for x16) or 16 GB/s (for x8) bandwidth.
- **Secondary PCIe Slot (PCIe x16_2)**: This slot might operate at PCIe 4.0 x4 or x8, connected through the chipset, providing up to 8 GB/s (for x4) or 16 GB/s (for x8) bandwidth, but with added latency due to the chipset.

In summary, the primary PCIe slot is faster primarily because it has a direct connection to the CPU and typically offers more PCIe lanes with dedicated bandwidth, while the secondary slot has to share resources and connect through the chipset, leading to reduced performance.

When testing with x86 guests it might use KVM so guest code runs on the CPU which might give different results. You can add -accel tcg to force it to translate x86 code the same way as it does PPC code which will be slower but might be closer to the PPC guest case. Although PPC and x86 ops are different so the translation is also different but at least it goes through more the same process as with PPC guest.