Generally for perf critical use cases you dedicate the machine to the database. This simplifies many things (avoiding having to reason about sharing, etc etc).
This makes me wonder whether there would be value in an OS that is also a DBMS (or vice versa). In other words, if the DBMS has total control over the hardware, perhaps performance can be maximized without too much additional complexity.
This is a bad idea from the 1960s: IBM TPF, MUMPS, Pick. As soon as the hardware changes it becomes slower and more complicated.
That was back when hardware was changing to a significant degree, though. Nowadays, there ain't really much that's new about hardware today v. hardware from 10 or 20 years ago - hence operating systems / filesystems being able to remain mostly stable instead of suffering from the exact same problem.