Big JSON files for one, with tens of thousands of rows. ST3 can load and regex replace nicely. Atom just was unresponsive.

To be fair, I forgot I got a new PC between the first time I used Atom and when I switched over to it.

However, I did try opening an 11 MB (JSON) file and it did indeed feel sluggish.

I'm using ST because I want to use the same editor for my code and data, and I rather frequently get 100mb-1gb textual data files, logs, tabular data in plaintext or json, etc - not big enough to require special tools to process (I mean, they're a small fraction of my RAM), so a text editor should allow to eyeball them and do some basic processing - edit a particular line, do interactive global regexp replacements to e.g. adjust the columns in a million-line file, etc.

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?

Many-megabyte, probably; think an assembly dump of your browser, for instance.

Not sure why you're being downvoted. Sure, maybe you're not going to have a need to open an assembly dump of your browser very often, but plenty of projects have multi-megabyte text/data files that need to be opened now and then.

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).