You say you just merged with another instance, how recently was that? Or the real question behind that, if you visit /sidekiq while logged in as an admin, are there lots and lots of jobs in the queue?
Importing posts is a multi-step process with most of the processing delegated to jobs that get queued and run after the initial import.
It sounds like you may have imported a very large number of posts and potentially it’s just still catching up.
An issue I also had with an import (from a legacy system wrangled into generic email import, rather than Discourse merge) was that thousands of jobs were failing and continuously retrying which slowed down the queueing for everything else. I forget exactly what that was but I was able to fix the problem in the post content which stopped those jobs failing.