I´m creating Groups that contains Sprites and adding bodies with box2D, but when I create many of this my animation gets very slow. How can I handle it?
Have a look at this if you're using spritebatch http://code.google.com/p/libgdx/wiki/SpriteBatch#Performance_tuning
And also if your bodies are static terrain and don't animate, then you should be using SpriteCache. Ex. you are building tile based terrain