What is the maximum read and write speed for pcie x4 slots?

What exactly does the spec “PCIe x16 slot ” mean? pcie 3 0 x4 slot 2 SSD is installed, the PCIe x4 slot will be disabled Also note how the specification suggests it is a full-size PCIe x16 slot, however it only supports up pcie 4 card in pcie 3 slot tl;dr: Has anyone tried an Apple OEM SSUBX SSD in the PCIe x16 Slot 2 of a Mac Pro 3,1 ? Does it negotiate PCIe

pcie slot splitter interface utilizes multiple technologies, including x2 to x4 PCIe Lanes, NVMe , SATA protocol, and USB