StructuredText processing can be pretty intensive. Naive uses of StructuredText can be pretty inefficient. For example, Zope's DocumentTemplate? has a structured text formatting option that performs structured text processing every time text is rendered.
StructuredTextNG splits processing into multiple processing steps. Assuming that one wants to be able to have DOM access to the structured information or produce multiple types of output, then the DOM version of the processed strctured text should be cached, rather than the final output. This means that outputter should be as efficient as reasonably possible.
This risk is mitigated in two ways:
Document the importance of caching intermediate results
The object itself could retain a cached version which is internally invalidated when its source changes, at the expense of some changes in the way StructuredText currently works.
Make sure that the outputter is implemented efficiently