When I see hybrid cables, I always wonder what does that means? Let say if silver has better conductivity than copper, then in a hybrid condition, will the electrons traveling on the silver portion of the cable goes faster than the copper portion? Will that create a coherence problem? And when we talk about metal's conductivity, what are we talking about? Does conductivity mean the speed the electron is traveling? Or the fullness of the bandwidth? Or certain bandwidth and that is why copper does better with low frequency and silver with high frequency? I have just purchased the Final Audio Design's Lab 2 and the stock cable that comes with the phone claims to be almost as fast as 90 percent of the speed of light and that it is used in supercomputer etc. So is as far as cable is concern, is the speed the only factor? Or does other factors are at play also, if so, what are the other factors? I am talking headphone cables and not power cables and under 5-6 feet. Also, for SPC, and since signal travels on the surface of the conductor, does it matter what silver is plated over? Like to hear some opinions from cable design people.
The "90% speed of light" claim, I think, refers to the speed of the electromagnetic field or the electrical signal. As for the velocity of the electrons, the only velocity of the electron, that applies for this situation is the Drift Velocity. A free electron in vacuum can be accelerated to very high speeds, but that doesn't apply here.
The value of drift velocity of electrons in a conductor, is several orders smaller than the value of the speed of light. Although the drift velocity can have a direct relationship with the speed of electrical signal, it is not a direct measure of the speed of the electrical signal flowing through the conductor. Meaning, Drift Velocity of Electron in a Particular Cable <> Velocity of the Electrical Signal in the cable.
So the 90% speed of light doesn't say much, unless described relative to the speed of the electrical signal, traveling in a regular IEM upgrade cable. For all you know, these regular upgrade cables might be conducting the electrical signal at, 89% or even 94% the speed of light. An electrical signal can travel at a speed closer to light's speed in a very-high-gauge ultra pure copper cable.
Conductivity is simply, the number of electrons flowing through the cross section of a conductor, at a given point of time, when a potential difference is applied across the 2 ends. Greater the number of electrons flowing through the conductor for a given potential difference, means the cable is a better conductor. As far as my understanding goes, this is the basis, from which the cable manufacturers work on, to improve cable's performance in the Headphone/IEM cable world and, not the speed of the signal itself, (although, both can be directly relative).
I know you have mentioned about signal traveling on the surface. But I won't talk about that as I am not familiar on that subject. But my text above is simply to say that, IMO, the speed may not have much to do, than the conductivity itself.