Wikipedia on velocity factor explains:
For optical signals, the velocity factor is the reciprocal of the refractive index.
On refractive index, Wikipedia also has an explanation of refractive indexes less than 1, which mean a phase velocity greater than $c$, but does not violate causality because phase velocity doesn't carry information. OK, fine. It even discusses negative refractive indexes, and while I haven't really absorbed what this means intuitively, I'll believe such materials can exist.
But now going back to the original statement, this would imply that there are materials with a velocity factor greater than one, or a negative velocity factor. I can't quite get my head around that.
Is this true? I'm not sure exactly what is meant by "for optical signals". Am I making an incorrect assumption somewhere?