I wonder how Acme[0][1], developed by Rob Pike for Plan 9 would compare. It feels fast and light, but I only ever use it for files lighter than a megabyte.
It opens a ~190 MB JSON file[0] in about 2 seconds on my machine. Scrolling and jumping around the file feels very smooth. Editing is also crisp. Writing the file back to disk takes about as long as opening it.
For test.xml the RSS is 20MB and opening it and going to the last character is instantaneous, search and replace on test.xml takes slightly less than one second.
[0]: http://acme.cat-v.org/ [1]: https://github.com/9fans/plan9port