To improve the performance of a large list or library, you can index a column. An index on a column enables Windows SharePoint to quickly analyze the data in that column, even when working with thousands or millions of items. For a view to quickly filter through a large number of items, the filter must be applied to a column that is indexed.
To improve the performance of a large list or library, you can index a column. Then you can use the indexed column to filter new or existing views of the list or library.
NOTE Each additional column index consumes extra resources in the database. Therefore, you should add indexes only to columns that will be used actively in views on the list or library.