The concept in databases that you’re looking for is called group commit. Transactions running concurrently batch writes together before flushing to disk. This means the minimum latency for committing a transaction is the speed of fsync, but the throughput can be as high as the disk bandwidth.

There are other reasons why an in memory database can run faster than a disk based database, such as not having a buffer pool manager, but I don’t think that’s what you were worried about.