The original post is deleted, but in a proper ACID database, persistence and speed are quite highly related. The DB can't truly move on until it has confirmed that a set of writes has truly hit the persistent storage, and the time required for that is vastly larger than the time required to process a transaction in volatile memory. We cheat in every possible way, notably with battery-backed NVRAM, so in actual practice the cost isn't always visible. But if you need truly persistent transactions, then fast persistent memory is a godsend. (Especially if it's big enough to put the WAL into. Though even a couple of bytes to store the last committed transaction ID can be useful.)