I noticed some years ago that vim can be much slower due to highlighting of particular syntax of huge files. For example it was fine with a 10MiB C file, but super slow with a 10MiB XML file, but fast again with ":syntax off"

Are you doing the formatting via a Vim plugin?

I often find it's much faster to pipe out to a command line program to do formatting on huge files.

e.g.

   :%! jq 

or

   :%! xmllint --format -

Huh, thanks, I didn't know you could put the contents on stdin like that (%!) - just in case you (or anyone) was similarly unaware in reverse, you can also put `%` wherever you want to use the filename in the command.

i.e. I would have done:

    :! jq %

and have probably been guilty of (not knowing `%!` per above):

    :! cat % | ...