Plug a passive guitar into a 30 ft cable and then into a 10 ft one. On most cables the longer run sounds a little duller and softer on the top. That change is the cable's capacitance at work.
What Is Actually Happening
A passive pickup has a high output impedance. Every cable has some capacitance between its center conductor and its shield, and that capacitance adds up with length. Put the two together and you have a low-pass filter. The more capacitance in the cable, the lower the point where the high frequencies start to roll off.
On a short cable the effect is small. On a long run, or with vintage-style pickups, it becomes easy to hear: less sparkle on the top, less definition on each note, and a slightly veiled sound overall.
Why It Matters More on Guitar Than on a Mic Cable
A microphone preamp or a line output drives the cable from a low impedance, so capacitance has much less effect. A guitar pickup has no such buffer. That is why capacitance matters more on an instrument cable than on almost any other cable in the studio.
What Low Capacitance Gets You
A low capacitance cable moves that roll-off point higher, out of the range where you hear it. The practical result is that a longer cable sounds closer to a short one. Your top end and your transients stay the way your guitar and pickups made them.
Ghost Instrument is Hi-Z rated and ultra low capacitance. It is built with Neutrik straight, right-angle and silentPLUG ends, hand-soldered in Los Angeles and covered for life against defects in parts and craftsmanship.
Related: How long should a guitar cable be?