That is true in a sense of: Of all the things in the graphics pipeline moving data between CPU and GPU is the slowest thing you can do. But that doesn't really

That is true in a sense of: Of all the things in the graphics pipeline moving data between CPU and GPU is the slowest thing you can do. But that doesn't really mean the PCIe bus is slow. The truth is that most machines have a single GPU, connected to the CPU with all 16 PCIe lanes but they usually hit their performance cap utilizing much less then that.