I use the strategy of preloading interstitials and rewarded videos at application startup, and this means that the user doesn't have to wait for an ad to load when it's time to display it. However, this also causes many ads that were requested to the admob network to not be displayed (because it was not the user's will or the time did not come), resulting in a low "show rate". I've seen people saying that Admob can penalize profits if the "show rate" is low, but I haven't seen it anywhere making that clear
