Deferring a layout update can be useful when you need to remove or add multiple fields in a report and you don't want to update the pivot table after each change. Now, you can update a pivot table after performing all changes at the report level in the field list resulting in better performance.
In this sample, the Defer Layout Update option can be enabled or disabled via field list UI.
In general, this feature can be enabled by setting allowDeferLayoutUpdate as true.