Thinking of upgrading from 2011 13 inch macbook air to the 12 inch 2015 macbook

You won't see a huge CPU performance increase. It's roughly equivalent to a 2011 MBA with a Core i7 There should be an increase in graphics power, but also keep in mind you'll be pushing more pixels.

https://www.google.com/search?q=macbook+12+2015+vs+macbook+air+13+2011&ie=utf-8&oe=utf-8

 

I'm waiting to see what they do with a 13" model.

 

You won't notice better performances with the Macbook.
The 12'' is a good choice if you need extreme portability, but if the main reason for upgrading is the retina display I suggest you to buy a 13'' rMBP. Of course is less portable than the 12'' but you'd have the same screen size with retina resolution and a really faster cpu compared to your 2011 Air.

 

I went from the late 2012 Air to the 12 Macbook and I find (whatever the numbers say) that the MacBook is snappier for basic web browsing, word processing, short videos (mostly news and a little music, etc). The weight and build are fantastic. If you are a writer just make sure you can live with the keyboard. That's the biggest difference. Many people get used to it. Some come to love it. But not everyone. I am very flexible about keyboards and I don't love it yet. I am typing this on a das keyboard plugged into the single port. Nothing like a das keyboard, but it isn't portable. Good luck.

 

My experience exactly, but I did get used to the keyboard. I bought my MacBook for travel but even at home I use it way more than my MBpro and have stopped using my MacAir and iPad altogether.