When should you shard a SQL database?

Hey everyone, I've been thinking a lot about how to scale up our database recently. Our main user table is starting to slow things down, and splitting it up, or sharding, feels like the way to go. But man, that sounds like a major architectural overhaul. What's your experience been like? When did you decide to shard your SQL database, and what were the hardest parts you ran into? Are there any specific numbers or signs you watch out for that tell you it's time?