On the face of it, I've got some really serious reservations about this - essentially it would appear to be the first enhancement in a while that doesn't scale at all.
My site has tens of millions of posts, and a couple of hundred thousand users. I am assuming any site-building is going to be a heavy and lengthy task. Given that our public facing servers can drop out at any moment, it's much better if we can run this on a non-public facing server kicked off by a unix cron job - but that doesn't seem possible with this 'enhancement'.
There are easy things that could have been done top reduce the load as well, such as limiting the results found, generation of video and image sitemaps, and combine it with introduction of structured data. I'm sorry but this improvement would appear to be a big disappointment compared to the other enhancements in 1.4 which are all great (except email bounce handling which is again let down by not being that suitable for very large forums).