Is interesting that the idea of micro services is throw like a obvious "solution".
Is not.
"Scale-up" MUST be the "obvious" solution. What is missed by many, and this article touch (despite saying that micro-services is a "solid" choice) is that "Scale-up" is "scale-out" without breaking the consistency of the DB.
Is a lot you can do to squeeze, and is rare you need to ignore join, data validations and other anti-patterns that are normally trow casual when problems of performance happens.
I don't know what to tell you other than I've seen vertical scaling hit its ceiling, several times. The OP lists "scale vertically first" as a given; to an extent, I agree with it, and the comment you're responding to is made with that as a base assumption.
There are sometimes diminishing returns to simple scaling; e.g., in my current job, each new disk we add adds 1/n disks' worth of capacity. Each scaling step happens quicker and quicker (assuming growth of the underlying system). Eventually, you hit the wall in the OP, in that you need design level changes, not just quick fixes.
The situation I mention in my comment was one of those: we'd about reached the limits of what was possible with the setup we had. We were hitting things such as bringing in new nodes was difficult: the time for the replica to replicate was getting too long, and Mongo, at the time, had some bug that caused like a ~30% chance that the replica would SIGSEGV and need to restart the replication from scratch. Operationally, it was a headache, and the split moved a lot of data out that made these cuts not so bad. (Cassandra did bring its own challenges, but the sum of the new state was that it was better than where we were.)
Consistency is something you must pay attention to. In our case, the old foreign key between the two systems was the user ID, and we had specific checks to ensure consistency of it.
Is not.
"Scale-up" MUST be the "obvious" solution. What is missed by many, and this article touch (despite saying that micro-services is a "solid" choice) is that "Scale-up" is "scale-out" without breaking the consistency of the DB.
Is a lot you can do to squeeze, and is rare you need to ignore join, data validations and other anti-patterns that are normally trow casual when problems of performance happens.