They added an option at "Settings > Core" to "Warn before opening files larger than this number of megabytes". I don't use Atom at all, but after so much discussion about this issue I don't think there is a better solution. This clearly shows the difference in performance of a web-based text editor versus one built on top of a native UI library, but people prefer to sacrifice that performance for the nice interface and the extensibility, I suppose that many of these happy Atom users don't work on huge projects like some of us do, Atom works okay for what it offers.