When should you consider moving from a single database to sharding?

hey everyone, im tryin to figure out the tipping point for when a single relational database just isnt cutting it anymore and its time to consider sharding. we're seeing load on our primary user table, and query times are startin to creep up during peak hours. we're not at the scale where its completely broken, but it feels like were approachin a cliff. what were the indicators for you that signaled it was time to plan for or implement sharding? were there specific metrics you watched, or was it more of a gut feelin based on growth projections?