I've never used it for anything quite that large. I have three windows open on my first virtual desktop: Editor, terminal, and browser. When I want to poke around in log files or other really long files, I'm using grep, tail, awk, and Perl (and vim), in the terminal.

The biggest file I regularly work with in the editor is 10,631 lines long (I just checked, and it's longer than I thought). Atom works fine with it, including syntax highlighting, and cutting/pasting/typing/etc.

Actually...out of curiosity, I just opened up a ~110,000 line file (a log I had laying around). It seems to work fine. It took a little longer to open than it would in vim (which is effectively instant), but not as long as I would have expected. It was well under a second, and within my comfort zone for opening files. Switching tabs seems maybe a little more sluggish with it loaded, but not uncomfortably so.

I think I have to accept Atom into the category of editors that can work with pretty large files. I can't imagine having a source file any longer than that 10,000 line file I work with regularly (and I would never make a file that large, but historic codebases sometimes have weird things in them), but it seems like it'll work even if someone did.

What do you use now?