Atom does not handle large files well at all compared to ST. I like Atom, but get very serious hangs with it on large files.
Notepad++ handled that surprisingly well. pretty much the only time i had to use another text editor besides ST
However, I did try opening an 11 MB (JSON) file and it did indeed feel sluggish.
I haven't tried Atom, but the Hydrogen plugin looks very interesting. How does it react to, say, being asked to do syntax highlight on a 100mb json?
One of programs I develop uses a dictionary that when unzipped is ~8Mb. Sometimes I need to open that up in an editor and it's nice not to have to fire up a different editor to do that.
In fact this is one of the first things I do when trying out a new editor - see how well it can handle files > 1Mb in size.
Maybe if you're only working on small projects with small files this isn't such a concern, but there are plenty of people who have legitimate concerns if an editor can't open large files (and 1Mb isn't even large).