I just ran a find over the project I've been working on full time, and the largest file, by a long shot, is 8kb, and a little under 100 lines. That's just an autogenerated enum of the HTTP status codes though, so it isn't indicative of real code.
In the vast majority of cases, all of my files are less than 4kb, and hover around 40 lines or so. Atom handles this use case just fine, and I'm hard pressed to think of a situation where I'd run into performance concerns. The entire process itself is using less than 80mb of RAM, and 0.2% of my CPU. A single tab of Chrome is worse than that.
What are people doing with their text editor that they need blazing speed?