Unfortunately, I too, have stopped using Atom in favour of ST3 simply due to the perceived responsiveness of the application. I don't have a lot of patience when I'm knee-deep in programming a new product feature to wait for files to open, delays in tab switching, etc. It's not massive, but compared with ST3, it feels like the UI is running through molasses.
I love the extra functionality that Atom provides, I love the package management, the theming, the community.. But I can't use it as my main editor when I still get this little annoyance all the time.
For those that are interested, I'm running it on a MacBook Pro Retina 15'