When should you consider adding a cache layer?

hey folks, we've been hitting some read performance issues on our main service lately. we optimized queries, added indexes, and even scaled up the database instance, but we're still seeing latency spikes. it got me thinking about caching. what are your general rules of thumb for when a cache becomes a necessity rather than just a nice-to-have? are there specific metrics you watch, or is it more about the nature of the data itself? curious to hear your experiences