Infinitely scalable's a bit of myth in my experience. We've switched firebase app to a small postgres instance and have seen all of our performance issues magically melt away.
Cloud Firestore how does that relate to Datastore? Really wish Google would allow multi-region Cloud SQL like AWS allows with RDS. Datastore is such a pain to use with its index requirements
Cloud Firestore scales just fine, like any database, you have to know how to work with it. You switched from a nosql database to a sql database... two totally different performance profiles.
Do you mean Cloud Firestore?
To be clear, when I reference the infinite scalability, I mean the ability of cloud functions to spin up as many instances as I need.
In terms of database load, I'd fully expect to have to migrate to a larger instance, but that is also relatively easy with cloud sql postgres.