Recently, while considering possible use cases for Delayed Durability, it occurred to me that data loss might be entirely acceptable in cases where the data would not truly be lost. I have worked with a number of applications that have processes that purge old information from the database. If a purge process failed in these applications, data would simply live a little bit longer, and be purged the next time the process was successful – they have a recovery mechanism built in as it is. I decided to test Delayed Durability in a database with a long-running purge to observe the potential performance impact. I chose a process that was clearly contributing to transaction log waits, because that is where the real performance impact comes from when delaying durability. If you do not have notable waits or some level of a bottleneck there, you are not likely to improve anything simply by turning on this feature.
I was not aware that you could set durability at the transaction level; I was under the mistaken impression that once you flipped the switch, all transactions were subject to Delayed Durability. Disk-heavy operations (like large batches of deletions) does seem like a good use case for this.