The best replacement for speed of light is sound.

The best alternatives to speed of light are rated as following:
★★★★★  sound
★★★★  speed of sound
★★★  electricity
★★  speed of electricity
★★  speed of thought
★  long arm of the law
★  speed of radio waves
★  speed of gravity
★  pursuit of justice
★  refractive index
★  wavelength
★  radio waves
  speed of bullet
  speed of dark
  time travel
  speed of lightning
  darkness
  thought
  light year

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Speaking about speed of light, one probably shouldn't forget about index of refraction.

Other search terms related to speed of light and their ratings are:
★★★★★ index of refraction
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speed of light
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