so, when should you switch from one database to sharding?

Hey everyone, right now we're using one PostgreSQL instance for our main app. It's worked okay, but during busy times, some queries are getting pretty slow. We're thinking about adding read replicas, but I'm curious about when we should really start looking into database sharding. What have your experiences been? What are the main signs that sharding makes more sense than just scaling up or adding more replicas?