Even with fibre (which is slower than the speed of light)
This makes no sense. Are you referring to the speed of light in a vacuum? Fiber transmits data using photons which travel at the speed of light. While, yes, there is often some slowing of signals depending on whether the fiber is single-mode or multi-mode and whether the fiber has intentionally been doped, it’s close enough to the theoretical maximum speed that it’s not really worth splitting hairs (heh) over
There are additionally some delays added during signal processing (modulation and demodulation from the carrier to layer 3) but again this is so fast at this point it’s not really conceivably going to get much faster.
The bottleneck really is contention vs. throughput, rather than the media or modulation/demodulation slash encoding/decoding.
At least to the best of my knowledge!
you’re often back-tracking across the continent before your traffic makes it to the end destination, with ISPs caring more about saving money than routing traffic quickly
That’s generally not how routing works - your packets might take different routes depending on different conditions. Just like how you might take a different road home if you know that there’s roadworks or if the schools are on holiday, it can be genuinely much faster for your packets to take a diversion to avoid, say, a router that’s having a bad day.
Routing protocols are very advanced and capable, taking many metrics into consideration for how traffic is routed. Under ideal conditions, yes, they’d take the physically shortest route possible, but in most cases, because electricity moves so fast, it’s better to take a route that’s hundreds of miles longer to avoid some router that got hacked and is currently participating in some DDoS attack.