I am building an app that involves storing statistics of professional baseball players, and am using MongoDB.  If I am concerned about lookup time, does it make more sense to have an individual collection for each player, with the dictionary data to be organized like {statistic_name : statistic}, or each statistic to be a collection with the dictionary data organized like {player_name : statistic} ? 
There will be significantly more players than there will be categories of statistics
In my opinion, you should make the first method : {statistic_name : statistic}.
It will be easier to select one of them. I think, i'm not 100% sure. 
Sydney
MongoDB collection design