Ask HN: Need help in identifying the tech stack I need

I am trying to build an app that will match readers based on books that they have liked. Essentially, whenever a new user is signed up, I want to go through the list of all users and give them a score depending on their match with the new user (2 points for each book match, 1 point for genre match) and then send the new user top 5 users with the highest score. Problem is this really sounds inefficient. What would be a good way of doing this? Should I look into graph dbs?