Fast workers are far better on quicker speeds, I'm not sure who thinks they are arguing what, but that's the statement I and every general consensus would stand behind.
That seems to be the consensus. I'm guessing (from statements like "1 Normal turn is worth more than 1 Marathon turn") that people are comparing Normal to Marathon like this:
A Fast Worker moves onto a hill and starts to mine. This is 1 turn faster than a normal Worker, netting you +2 Production when it's finished over a Worker. The player is currently building, say, The Pyramids. At Normal Speed, that +2 Production is worth 3x times as much when building a wonder as it would be on Marathon (where wonders cost 3x times as much to build). So that's +6 Production on Normal vs. +2 Production on Marathon. A clear advantage.
But the game doesn't work that way. Both players play at the same speed. So the +2 Production from the Fast Worker is worth exactly the same on every speed setting: +2 Production.