Have a link that discusses this?
Cory House
2

It’s kinda like defining your functions inside the render method. You don’t need to worry about binding any more, but you create new functions every render, even if they are not used. And since stateless functions provide no way to provide a shouldComponentUpdate method yet, you can’t prevent unnecessary re-renders creating new functions. It’ll probably be fine for normal apps, but will hurt the performance badly for larger lists.