The support for large files leaves MUCH to be desired and the code to address this seems to be in core (which is currently closed source), which prevents addressing myself.
It is often the case that I wish to open ~100MB log/csv/etc files for a quick search or edit. As it stands now, Atom is completely unusable for these scenarios. Large files (e.g. ~100MB) refuse to open at all, small-ish files (e.g. ~5MB) also refuse to open, ~500KB files open but lock ups are common.
Here are a handful of geonames files that can be used to test: http://download.geonames.org/export/dump/