Is your 2012 MacBook Air running hot, with the fan blowing like crazy and Activity Monitor showing a higher percentage of CPU usage by the Dock? If so, the fix might be an easy one.
On mine, I was using some customer photos as wallpaper (specifically, a random selection from a DropBox folder). I don’t know if it was the photo itself, or the process of selecting a random image, but once I switched back to a stock photo my MacBook Air returned to normal. The fan spun down, temps decreased, and the Dock joined other lower-resource apps in using just a sliver of the CPU’s time.
That seems like something that Apple might want to fix sometime soon, but it’s good to know that the fix might be so easy. If this works for you, please let me know in the comments.