6Gb(it)ps of a maxed out SATA III link is still not slow, it's just that the multi-GB(yte)ps speeds of modern PCIe drives is just insanely fast. My TrueNAS box has 3x2 mirrored vdevs of 7200RPM drives and has plenty of bandwidth to handle multiple 4K ProRes streams, the only reason I have PCIe storage (an Optane 800P) on it is because I don't want to waste a precious 3.5" bay for a SLOG device to handle sync writes (VM workloads).