If you primarily look up by the primary key which is pretty common the clustered index is faster than looking up the primary key in a standard index then findin

If you primarily look up by the primary key which is pretty common the clustered index is faster than looking up the primary key in a standard index then finding the row in the heap for the other columns. If you make the standard indexing covering all your columns you have the behavior of a clustered index except the data is duplicated in the heap. Clustered indexes can save significant space on a narrow table with a lot of rows accessed in a particular way and perform better as well, if you access the table in multiple ways with other secondary indexes it can be slightly slower to access through a secondary.