Two PCIe slots looked the same but gave different benchmark results - PCIe-2 vs PCIe-3

I benchmarked a new-old NVMe disk drive I got off eBay and didn't get the performance I expected in a benchmark. It turns out I used the wrong PCIe slot. All the connectors looked the same in the dark. The old HP Z820 machine has a bunch of PCIe-3, apparently one PCIe-2, and one classic PCI slot. PCIe 3.0 supports a max speed of 8.0 GT/s while PCIe 2.0 supports a max speed of 5.0 GT/s. I have no idea if there was any real-world difference. PCIe 2.0 vs PCIe 3.0 These are two representative samples with an Intel P3700 in the v2 vs v3 slot. I ran both of these with the Default setting not the more stressful NVMe CrystalDiskMark configuration. There is a pretty significant boost in sequential performance moving from PCIe-2 to PCIe-3. The rest of the settings appear to be maxed out because of the test configuration. Test Configuration Impact - Saturating the Device CrystalDiskMark 8 has two different test modes, Default and NVMe . The difference is the number of Queues and the n