VIew Index Filter

Index the Content Type column

In the scenario where you have a library with more than 5000 items comprising e.g. 1000 folders and 4000 document items, you will still get the threshold error even though the number of documents is less than 5000. In order to get all of the threshold (5000) applied to just documents (not folders) you can index the content type column. To do this go to library settings, then indexed columns as below:

Then select Create Index:

Then select the Content Type column: