Paul White explains the performance benefits of fast key optimization and when you can get them:
SQL Server can be called upon to sort a variety of data types. To facilitate this, the sorting code normally calls out to a specific comparator to determine how two compared values should sort: lower, higher, or equal.
Although calling comparator code has low overhead, performing enough of them can cause noticeable performance differences.
To address this, SQL Server has always (since at least version 7) supported a fast key optimization for simple data types.
Click through to learn which data types support fast key optimization and to gain a feeling of the performance impact.