Denser medium has more 

Velocity of light (V) is inversely proportional to refractive index (η), where proportionality constant is speed of light (C)

⇒V= Cη

So, if the refractive index increases the velocity of light decreases.

So, the velocity of light in denser medium is less than the velocity of light in rarer medium.

Therefore, denser media have greater refractive indices but lower light velocities.