Using Sethblings MCEdit filters, I combined a firework and a minecart into an entity in which the minecart rides the firework, then attempted to load the game with it in. Game loads fine until it attempts to do anything with the entity I created. Deleted it using MCEdit, map loads fine with standard FPS. Tried spawning it in again, get less than 1 frame every 5 seconds... Debug Screen (F3) displays standard FPS readouts, even though FPS is less than 1.

In case you're wondering, I wanted to make an epic rocket.