While you are mantling, you are able to execute a Mantle Glide. Mantle Glides are an intentional mechanic where you are able to gain speed out of mantling by holding down the crouch button while mantling up a cliff.

To execute, hold crouch while mantling. The timing is extremely lenient; as long as you are holding crouch as the mantle ends, you will be able to Mantle Glide. By holding crouch after the Mantle Glide, you are able to slide.

A Mantle Glide will typically have an outgoing speed of ~550 u/s. This can be lower depending on the type of surface you are mantling on (e.g. a steep curved surface).

Mantle Glides are incredibly useful for early game traversal and escape. Mantling by itself would usually kill and stop your momentum, allowing for opponents with superior movement abilities, items or attributes to catch up to you; with Mantle Glides, it can at least give you momentum coming out of the mantle, or even start a movement chain.