Improve How SharePoint Online Performs - Creating a Document Library Index

Indexing columns can be very useful when organizing several items in a list or library. To improve the performance of a list or library, you can index a column. You can also use the indexed column to filter views. Each additional column index consumes extra resources in the database. So make sure to plan ahead and only index the columns that are needed.
Creating an Index on a Column in Your Library

1. In your document library, select the "Library Tab" in the Ribbon, and then click "Library Settings".

1. Under the Columns section, click the "Indexed columns" link.

1. Then the Indexed Columns window appears.
2. Click the "Create a new index" link.

1. In the Primary column for this index, select a column that you would like to index.

1. Click Create.
2. An index has now been created. This index will be available for the Filter by option when creating a view.

If, you are interested in learning more you can view this article https://support.office.com/en-nz/article/Manage-lists-and-libraries-with-many-items-fba0c8b7-79bb-4ea4-9aff-bf145a689e8e