Investigate the performance impact of __wt_directory_sync in workloads that do a significant number of those operations (e.g., insert workload with small log files, or an LSM workload with small chunks).

If the directory sync is expensive, consider making it optional (presumably with a wiredtiger_open config directory_sync=true/false). The standard says that some filesystems may require it for durability, but in practice the most commonly used filesystems don't.