[k0stas]

I have designed PCIe compatible transceivers and this will be the last comment I make about it because correcting hardware nonsense on HN is only of transient interest to me.

Everything I wrote is 100% correct and in fact incontrovertible. Whether there is a little or a lot of differential capacitance does not change the fact that the differential portion of the capacitance has a 2X effect on the differential signal (as opposed to the common-mode signal, which it has no effect on). This is supported by basic math.

If capacitance is to ground then it is not differential capacitance so it is not relevant to this discussion. It may be true that differential capacitance is not a significant contributor to the impedance of PCB differential traces but that does not change the fundamental result (similarly, the principle of photovoltaic conversion still holds true in the dark even though there is little light to convert). And PCB traces are not the only kinds of differential pairs. Diff pairs exist inside the integrated circuits that drive the PCBs where they operate less like transmission lines and more like lumped capacitances due to the frequencies of interest compared to the dimensions of the conductors. In these circuit and conductor structures, differential capacitance can be significant and this is what OP was talking about since he was talking about the legs of the driver (transistors). OP was just wrong about the differential capacitance being good for speed. It's bad for speed.

[CamperBob2]

Meh. There isn't a whole heck of a lot of capacitance between two parallel traces. Not compared to a single-ended trace that is (almost necessarily) referenced to one or two planes on adjacent layer(s).