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It's worth noting that the PCIe standard is backwards compatible, so you can, for instance, use a PCI-e 4.0 graphics card in a PCI-e 3.0 slot without issue, although you will see a small performance hit.

a PCIe 4.0 x16 slot can theoretically hit approximately 32 gigabytes per second (GB/s) of data flowing in each direction, while PCIe 3.0 maxed out around, you guessed it, 16GB/s.

A typical x4 NVMe SSD slot on a PCIe 4.0 motherboard can achieve a total data transfer rate of up to 64 GB/s. ... PCIe 4.0 slots offer higher data transfer rates compared to PCIe 3.0, resulting in improved NVMe SSD performance.