Gen1 to Gen2 doubled the clock rate from 2.5 Gb to 5.0Gb. Gen2 to Gen3 increased the clock to 8 Gb, but also reduced the encoding overhead (from 8->10 to 128->1

Gen1 to Gen2 doubled the clock rate from 2.5 Gb to 5.0Gb. Gen2 to Gen3 increased the clock to 8 Gb, but also reduced the encoding overhead (from 8->10 to 128->130). There were some tweaks to the electrical level of things as well. The electrical level isn't too magic, but there are still a lot of things that have to be tuned (the chapter on tuning in the Mindshare book on PCIe is about 100pp). For a commodity consumer bus, the relative reliability and speed of PCIe is kind of a miracle.