Why dispersion of light is not observed in glass slab but it is observed in prism?

When a light passes from one medium to another, at one interface, it changes its speed. The glass slab and prism both have two glass air interfaces.

Hence, the light undergoes refraction twice in both the cases. When the two interfaces are parallel to each other, although the colours are separated at first interface, they all travel the same path after refracting from second interface. However, in prism, the two interfaces are not parallel. Therefore, the colours separated at first interface do not travel the same path after second refraction but emerge out at different wavelengths producing spectrum.