Unused areas in navmesh. Are they bad for performance?

When I generate a nav mesh to my level it generates it on the whole terrain or what ever I am using to walk on, including the areas the player is never supposed to and can't even go to. This leads to multiple nav mesh "islands" that have no use.

Are they expensive in performance?

If they do result in significant performance issues and it is worth doing something about, how do I get rid of those "islands"?

Thanks in advance :)

Edit: I know I could just put a "considerably huge amount" of static meshes with "Not walkable" on and hide them before playing the game, but is this really the only way to go with it?