When is it worth the complexity to shard a relational database?

Hey everyone, I'm working on a project where our main relational database is starting to slow down. We think we'll have a lot more users and data soon, so we're looking at sharding. We've tried indexing and making our queries better, but we're wondering when sharding a relational database is actually worth all the extra work and complicated code. What signs or numbers do you check that tell you it's time for sharding? And what problems should we watch out for, besides just getting it set up? We're trying to figure out if it's too early to even think about this, or if we should start making plans now.