Who cares about start-up time? I only open the thing once/day. Same thing for large files: it's a edge case. Memory usage? Sure, but as developers we probably have GB and GB sitting there, if the thing worked it wouldn't be a big deal for the most important tool in your arsenal.
What Atom sucks at is regular file editing: huge latency when typing, constant freezes (try editing remote files), and general sluggishness.
What they'd need is a huge architectural overview, and the fact that they're showing off how much they're optimizing proves that they'll never be as performant (or even close) as they'd need to.
Also weird that they blame extensibility. Emacs does it, Firefox does it, Chrome does it, and most importantly VS Code does it. I'm sure it's not easy, but it's done by other projects.
Too bad, because the UI is very nice, package management is very good, as well as not having to edit JSON files for settings (which sure I can do, but I see no added value and I'm lazy).