Comment by radu_floricica

To note that quite a bit of the performance problems come when writing stuff. You can get away with A LOT if you accept 1. the current service doesn't do (much) writing and 2. it can live with slightly old data. Which I think covers 90% of use cases.

So you can end up with those services living on separate machines and connecting to read only db replicas, for virtually limitless scalability. And when it realizes it needs to do an update, it either switches the db connection to a master, or it forwards the whole request to another instance connected to a master db.