The goal of PCI Special Interest Group is to double the data rate of PCIe every three years and it’s been true to this since version 1.0 in 2003. Version PCIe 6.0 has been finalized and data rates top out at 8GB/s per lane, meaning a heady 128GB/s on a fully-wired x16 slot.
It will take a while to filter down to the desktop, where we’ve just got used to PCIe 4.0. It has been achieved using PAM4 encoding (Pulse Aptitude Modulation), with four signal levels rather than two, so you can carry twice the data on the same signal frequency. PAM comes from the world of high-speed networks with fewer physical wires and little headroom to increase the frequency. –CL