Yosemite is a total mess, but it has gotten slightly better with 10.10.3 (if you are willing to ignore the Photos app, which launches constantly for no reason).

If you are willing to mess with drivers, you may want to try installing Linux. On a 2013 Air, battery life and performance (even graphics!) are surprisingly better with Debian Jessie than with OS X. Of course, it is a newer model than yours, but the drivers will only be more stable and mature with a 2011. Especially if you have 4 GB RAM and you're running into a memory barrier with OS X, you should see a significant improvement.