For conventional relational databases like Postgres, multiple nodes only give you reliability, not performance (ignoring things like read-only replicas which your application explicitly has to choose to query), so horizontal scaling doesn't help there.

Enterprise-grade SSDs is what I meant by direct-attach storage - I was comparing against network storage which is what all cloud providers use (your EBS volume is accessed over the network internally, which incurs some latency, ultimately limits IOPS and destroys random access performance which can't be cached or read-ahead by the underlying hypervisor).