I base my belief that [[wikitopics]] is operationally faster on a basic difference between the two designs, as I think the wikipedias will operate faster if they merely transclude infoboxes of their choice, at their own speed, from the wikidata central repository.

Transclusion is surely fundamental to wiki application design. The [[wikidata]] proposal by contrast is a client-server API, such things an artifact of the 20th century. What is the point of it here?

Ultimately the problem you're grappling with is not just just about infoboxes, it's about *anything* other than article text that has multilingual requirements. For instance, the same *pie chart* is to be shared among wikipedias, the only difference being the graph's title, key and other labels... [[wikidata]] is today doing format=table, later other formats. That's alot to handle in an API. 

So, it's highly advised the client-server API approach be scrapped. At a minimum, it's outdated technology, for good reasons. Instead, wikidata should *publish* infoboxes that are happily cached on wikidata servers. That's the best performance that can possibly be had.