A node access rebuild immediately removes access to all nodes on the site, then goes through each one individually and basically node_load()s and then node_save()s them. This can take an incredibly long period of time on a site with tons and tons of content. And because the query goes from oldest to newest, the site is stuck with old crappy content showing up in views and stuff until the node access rebuild is complete.
It seems pretty obvious that it should go newest to oldest instead so at least while most of the content was invisible it would be old crappy stuff no one cares about.
I was surprised not to find a pre-existing issue about this. I might've missed it.