Imho slow is one thing (I could probably even live with the performance) - the other thing is that font rendering in Atom is horrible. At least for me on Windows the text is blurry as hell. No comparison to the clearness in Sublime Text or other native editors (Visual Studio, Eclipse, Notepad, ...).

Brackets is better in that regard. Both brackets and Atom provide better extensability APIs than Sublime - but that doesn't help when the performance is worse than complete IDEs.