Hmm ok. Resistance/type of medium does though definitely....where does the 80% of the speed of light measurement come from btw? Quite interested in this topic!Greater impedance won't slow the electricity down!
An enhancer must be a necessity for live shows. If you could imagine at the average festival it would surely not be uncommon to have several miles of cable and various speakers hundreds and thousands of metres apart which would surely present a big muddy wall of noise were some delay based trickery not used.
We use other delay based effects in pro audio too. You've probably heard of a Phaser ( 0.1ms Delay) Flanger (5-15 ms Delay) and Chorus (20ms delay)
These are seriously microscopic times, but each effect is very distinctive. A Phaser is basically a 10,000th of a second delay and is of course very noticeable, so I'm guessing 1/40,000th of a second also would be very noticeable too, I wonder if the human ear is much more responsive than we are giving it credit for in this calculation?