gpu stuck in pcie slot - Do all Standard-ATX sized motherboards have a 3-slot ... samurai 888 katsumi slot machine

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I figure I should use the PCie 3.0 x 8, but would it slow down my GPU in the PCie 3.0 x16 slot?

If you purchase a PCIe 4.0 GPU for your system and seek to benefit from PCIe Gen 4's performance increases and reduced latency, then your motherboard will need to be populated with a PCIe 4.0 slot of appropriate lane size to support your PCIe 4.0 GPU.