The mesh renderer component of a game object can be viewed in the Inspector panel. There are multiple settings that can be adjusted to enhance performance.

The Cast Shadows setting can be set to On, Off, Two-Sided, or Shadows Only. The default is On, so you should disable this for all objects that do not need to cast shadows. 

The receive shadows is a toggle that tells Unity whether you want that object to receive shadows or not. As you would expect, this takes extra processing to display during the game. So, if you do not need an object to receive shadows, deselect this for greater performance.