Tested with a real XML file here:

# wc -l t.xml 315479 t.xml

# wc -c t.xml 10377128 t.xml

Unlike Sublime, it doesn't syntax highlight for some reason, but it does open. Opens quite fast, too. Editing, cursor movement, pagination, etc. all super fast.

OS X 10.11.3, Atom 1.6.0 on a MacBook Pro (Retina mid-2015, 2.8GHz Core i7, 16GB RAM, AMD Radeon R9 M370X).

> Unlike Sublime, it doesn't syntax highlight for some reason, but it does open.

Perhaps Atom is disabling syntax highlighting for sufficiently large files?

Edit: Interesting experiment. Open a small xml file, and syntax highlighting is active. Open a large xml file, and Atom doesn't syntax highlight. Copy and paste the contents of the large xml file without highlighting into the small file with highlighting, and after the text is copied there is a massive slowdown as Atom syntax highlights. I was able to scroll down the file and watch the syntax highlighting slowly propagate through the file.

Interesting -- disabling the xml plugin significantly improves performance for me (but I lose syntax highlighting).

Do you mean the built-in "language-xml" package or something else? FWIW, I have it enabled.

> Do you mean the built-in "language-xml" package or something else? FWIW, I have it enabled.

Yes, I'm using the built-in language-xml version 0.34.2.

I sure hope it doesn't try to highlight the entire thing, and instead just what is visible on-screen.

With most languages and things like XML it will need to have parsed from the start to the visible area to understand the context (e.g. what if there is comment that started 1000 lines earlier but hasn't yet been closed).