Microsoft SQL Server 2014 Unleashed

Clustered Index Indications

Searching for rows via a clustered index is almost always faster than searching for rows via a nonclustered index—for two reasons. One reason is that a clustered index contains only pointers to pages rather than pointers to individual data rows; therefore, a clustered index is more compact than a nonclustered index. Because a clustered index is smaller and doesn’t require an additional lookup via the row locator to find the matching rows, the rows can be found with fewer page reads than with a similarly defined nonclustered index. The second reason is that because the data in a table with a clustered index is physically sorted on the clustered key, searching for duplicate values or for a range of clustered key values ...