Don't use util.format as it's superly duperly not performant

[SCdF]

See the docs: https://nodejs.org/api/util.html#util_util_format_format_args

Please note that util.format() is a synchronous method that is mainly intended as a debugging tool. Some input values can have a significant performance overhead that can block the event loop. Use this function with care and never in a hot code path.

Using this function causes json2xml to be super duper slow with larger json structures. One that is ~70kb takes ~8 minutes to convert on my laptop. Switching a different library that same JS converted in milliseconds.