After a successful import, I've tried to rebuild the search index. Yesterday I started it pretty late and forum had around 60 active users at that moment. It started nicely but came to a crawl couple of hours later after indexing 1,5M messages.
This morning I cancelled it (it had made no significant improvement from 1,5M overnight) and started again, this time with a delay of 1 second. Again, it started nicely (this time forum was almost empty) and now it's taking 20 seconds per 500 messages.