The really fasted XML editor I've ever used is XMLMarker which loads a 5MB file in a fraction of a second. When editing, it shows the XML simultaneously in tree, text and 'Data Grid' views which is quite impressive as are a couple of its other features:
- You can edit data in the data view, not just in the text view
- If you 'break' the document by editing the markup in text view, the data and tree views still work (arguably a dangerous feature: caveat user)
But I'm often doing RegEx search and replace, which XML Marker doesn't do, and often editing text as well as xml. Notepad++ is quite speedy and can do regexes, but Sublime Text 2 does regex search and replace much better: it does live highlighting of what its found as you type, so you can 'debug' your regexing on the fly.
Everyone wants xml pretty printing (well, re-indenting), which for Sublime Text 2 is done by first installing the WBond.net package manager and then installing the 'Indent XML' package. Package Management in Sublime Text is a bit command-liney, so do read Mr. Bond's instructions.