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.
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).