I know partially the answer. But still I am confused. Auto layout, size classes these things are important to build the app. But what are the other things we need to care to implement an iPhone app vs iPad apps.
Mostly autolayout and size classes.
Performance wise it's pretty much the same with newer iPhones. Just need to pay great attention for the enormous number of pixels on the new iPad. I also recommend to make your app controls bigger and closer to the edges in iPad app, this will greatly improve the user view about the app