When would you bother building your own database,

we're running into some super specific performance problems with our current relational database, you know, for this one thing we do that needs to be really fast and handle tons of requests. it's got me thinking, should we just build our own data store for this service. has anyone actually done that. what made you do it, what was the biggest shocker, and how do you even figure out if it's better to build it yourself or just buy something for something so important