strategy for returning JSON from XML canonical model

Conversion from XML to JSON should be relatively light, but the mere fact it has to do something will take overhead. Doing that work upfront will definitely save time. You can put both formats in the same envelop (though JSON will have to be stored as string then), or in a different document as you suggest. Alternatively you could also store it in document-properties. Unfortunately, that only takes XML as well, so you will be storing your JSON as string in there too.

Alternatively, have you profiled the transform to see if there is a particular reason why it slows down so much? Using XSLT versus XQuery for the transform could make a difference too..

HTH!

json:transform-to-json has 3 algorithms optimized for different purposes and will perform with different tradeoffs of flexabilty, fidelity and performance.
"basic" (default) useful only to reverse json:transform-from-json()"full" - to preserve as much information fidelity as possible, in exchange for a non 'prety' format in many cases."custom" - is ... custom ... designed when the json format is fixed or when you want control over the json output at the expense of handling a subset of XML accurately.

Basic and full are the most efficient. However all variants are fairly involved and require completely traversing the XML node tree and creating bottom up a JSON object tree. In ML version 8 this is then translated into the native JSON node structure. In a REST call it would then be serialized as text.

Compared to a direct return of an xml document vi fn:doc("file.xml") there is atleast 2 orders of magnitude more operations involved in the transform case.For small documents in a REST call that still a small fraction of the total request time, expecialy if the REST call was performing a complex operation itself then returning a small result. Your use case seems the opposite - returning a xml document directly bypasses almost all of the XQuery processing and is sent directly from internal to the output or assigned to a variable.

If that an important use case to optimize, especially if the documents can be large, then saving them as text or binary will be much faster -- at the expense of more storage used. If this is only a variant representation of the xml, try storing the text JSON as binary as it will not incur any indexing overhead.

Otherwise if you need to query over the JSON then in ML7 storing as text gives you simple word queries, in ML8 storing as native JSON gives you structured queries -- both with efficient text serialization.