I’ve been a little freaked out by this lately too. Here’s what I realised today:
The output in a rails s terminal will show a bunch of querying on start-up, but then it also includes output from discourse’s sidekiq jobs (at least it does in the docker dev set-up). So if you’re running a really big import, then you’ll be getting a really a big sidekiq queue of post-processing jobs which might run for ages afterwards. I think these are non-essential jobs e.g. loading caches, which don’t seem to prevent you browsing the site before they’re finished.
This was confusing the hell out of me because I did a big import, then backed out of that and re-initialised the database to do a tiny test import, and it was still endlessly churning away doing querying related to the no-longer-existing big database! The solution was to empty the sidekiq queues using a rails console.
Too late to help you, but thought I’d share that.