Parsoid has the ability to drastically reduce API load and slightly reduce parse times by reusing image and transclusion HTML from previous HTML versions. When enabled, this eliminates basically all imageinfo requests, and a large proportion of expandtemplates requests, along with parsing of responses.

The implementation used to rely on the original wikitext source (data_parsoid.src) as a key. This property has been removed a few months ago, which implicitly disabled all fragment reuse.

Fixing fragment reuse by providing an alternate key is likely to yield a very good cost / benefit ratio. I think it is worth prioritizing for this reason.