They have a typical speed of around 12 m/s, though it is by no means entirely universal. You could certainly imagine a crowd being instructed to go much faster or slower when doing the wave, and though I never done seen it I don’t think it would be very difficult.
If you had a truly enormous imaginary stadium, maybe one with hundreds of thousands of miles of bleachers and an audience to match, you could run some interesting demonstrations. I pose a thought experiment: how fast can you get the wave to move in this stadium? Speed of sound? Speed of light? Faster? If faster than light, is this necessarily a violation of relativity?