Comment by dapperdrake

Firstly, atomicity and/or thread-safety not composing is where the Consistency and Isolation come in.

The "application layer" always has to enforce its own consistency guarantees. If the lower layers are total garbage, then the system is garbage. And the "speed" of the lower layers can be infinitely fast and it doesn’t matter, if the application has a latency floor. So optimize it all you want.