Because of the way ads work under the hood, it is possible for them require non-trivial CPU resources in the Unity engine. This is especially true for ads that incorporate advanced features like animation. For this reason, we generally recommend showing banners on menus, pause screens, and so on rather than directly in-game.
As Albert suggested, Interstitial ads are a great alternative to always-on banners. Try our
best practices page for more info.
-Andrew