when do you bother focusing on read speed versus write speed

Hey everyone, I've been thinking a lot about performance recently, especially with microservices. We always say we want things fast, but I'm wondering about the give-and-take between focusing hard on read operations versus write operations. For instance, if we're making a social media feed, reads are obviously a huge deal. But what about systems where writes are the main thing, like a busy transactional system or even a logging service? When do you choose to focus resources on making reads faster versus making sure writes can keep up? This is especially tricky when you think about caching and how you design your databases. What do you all think and what have you seen?