Light slows down in waveguides too. A heuristic explanation is that as it travels down the guide, it's also bouncing off the sides, and so effectively going at an angle.
The maximum signal speed loss due to inclination would be sqrt(1+(1/refraction_index^2))-1, or around 20% for fiber described above. Although, in practice, the fiber would not be even close to its maximum angle.
But potentially, a lower refraction index fiber with a reflective coating would allow signals to travel faster.
