While working to reduce dom size to improve VE load performance, we cleared out a lot of information that used to be lingering around in the HTML unnecessarily. One thing we cleared out is the "data-parsoid.src" attribute for extensions and transclusions since it is no longer used during serialization and it was considered useless baggage.
However, it turns out that template/extension reuse cache is keyed on the "data-parsoid.src" attribute => since the time we deployed code to reduce DOM size, we have not been reusing these expansions from the cache.
See https://github.com/wikimedia/parsoid/blob/10392facfc68bd821273892507343af7f42c4844/lib/mediawiki.DOMUtils.js#L1569-L1591
We need to fix that code to build the cache key from other available information (or if not possible, revive dp.src, if this is considered critical for performance).