Maybe Sublime has spoiled me, but that editor is pretty fast. It can open large files. It's almost as extensible, especially for any kind of practical purpose. Oh and it's cross-platform, which Atom is not at the moment.
I also can already see a time when we reach Atom package hell. Someone is going to have to come up with a system to manage all these packages pretty quick. When you are literally installing packages to add a single hot-key, and all packages are treated as equals, it quickly becomes very difficult to find what you are looking for to manage your install.
I'm excited and optimistic to see where Atom will go, but my thoughts before I tried it are only reinforced after a few days of use. GitHub does not mess around. I know they will fix most of my concerns pretty quick, but I'm not sure how much more can be done about performance.