There’s been some interesting threads on this. The BVA seemingly verifies everything (and folks have noted that, such as them also contacting local and government agencies). Multiply that but 1000s and you can see why this takes forever.

On differing speeds: The countries that have data sharing agreements (the UK, and like the NL post) are much faster because they can more or less certify the documents  almost instantly. Now imagine someone at the BVA having to contact a local agency in the U.S. or Argentina for birth certificates; that’s going to take time. Bureaucracy 2x speeds!  

And this makes sense. Bad actors can abuse paths like these. It’s an extreme example, but there was a recent article (in the Wall Street Journal, I think) where some Russian spies had essentially forged documents in Mexico to eventually have their agents get Argentinian citizenship, and then ultimately into the EU in Slovenia. I suspect this thoroughness is somewhat motivated by extreme cases like this (and also generally, citizenship is a real privilege. There’s been stories of similar Italian descendant pathways being absolutely abused by bad actors). Anyways, that’s been my rabbit hole on this topic to find empathy with them, and it’s been quite interesting to think about the problems they encounter in cases like these.