I currently have 10Tb of 1080p content and rescans takes a few seconds. The only thing that actually takes time is the intro detection on new content, but it's because I don't have a lot of compute power on the server hosting my Plex instance.

I just checked: my "com.plexapp.plugins.library.db" is 24Mb. Meanwhile, I have some custom apps with SQLite db's that are >10Gb and I don't have performance issues.

So I don't see how big a library have to be for its db to become a bottleneck?