Every fast write moves work somewhere else

Every storage engine has to decide what must finish before it tells a client that a write succeeded. The quickest answer is to return after copying the bytes into memory. A local durable write waits for fdatasync() on an SSD in the database host. Keeping the write after that host disappears means waiting for a network volume, an object store, or several database servers to save their own copies.
Those choices move latency and durability together because returning after memory is fast, but a machine crash can lose the write.