I think 0.18.3 fixed some of it, but there are likely some more performance issues related to PostgreSQL lurking in Lemmy.
A TRIGGER in SQL is a logic that executes based on other activity.
Lemmy uses them so that when you create a new comment or post, it executes code to insert tracking record for votes and comments on a post. One of the things Lemmy does is called site_aggregates, and there was a bug where it was updating the counts for 1500 servers instead of just the one server. That got fixed in 0.18.3
Deleting accounts in lemmy was causes crashes. I'm not sure if that has been entirely resolved. These things are all kind of hidden in the background of the code, so a lot of developers overlooked that there were problems in them.