100 feet / speed of light = 101 nanoseconds, round trip that's 202 nanoseconds, but electricity is ~66% of speed of light ~= 300 nanoseconds or .3us. (Fiber is also ~1/3 slower than the speed of light both because it's not a vacuum and the path is not strait.)
Now 10 us vs 10.3 us might not sound like much but it's still 3% slower. And it get's worse when you look at high end DRAM based SSD's which can be faster than 10 us.
According to Wikipedia, it depends on the insulation:
== SNIP ==
Propagation speed is affected by insulation, so that in an unshielded copper conductor ranges 95 to 97% that of the speed of light, while in a typical coaxial cable it is about 66% of the speed of light.
At data rates, transmission lines are used in copper. This is why CAT-5e has higher requirements on the twisted pairs than CAT-3 (or plain old phone line).
66% turns out to be a surprisingly consistent approximation for both copper and fiber.
Now 10 us vs 10.3 us might not sound like much but it's still 3% slower. And it get's worse when you look at high end DRAM based SSD's which can be faster than 10 us.