I've found that when pasting in huge files, disabling syntax highlighting makes a big difference. In fact, if you simply open a sufficiently large (I haven't bothered bisecting the size, but a few megs works) xml file, Atom will disable syntax highlighting by default and the performance will be slow but still usable. If you paste in the contents of a large xml file, syntax highlighting is not disabled and the editor either hangs or crashes, in my experience.