I think today, most often than not, the fault is between the keyboard and the chair. There is clear lack of domain knowledge. RDBMS's like PostgreSQL, Oracle, DB2, etc. can do much more than people expect them to do.

Relational databases are relatively easy for anyone to start working with, but optimizing and tuning queries requires fair amount of domain knowledge. You'd be surprised to learn how little denormalization big financial company databasees have. They have people who are experts that do only databases.

For I/O intensive work you may need transaction rows where data written together is stored together, but it's not denormalization, it's optimizing for I/O bottleneck.

Query planners can do most of the work, if the person crafting queries has understanding of how they work. Carefully grafted queries and tuning memory layout for tables correctly is usually the right way to fix performance problems.