In Salesforce Lightning, If we have detail pages that have a large number of related list records, we might be experiencing page load slowness and usability issues.
Advantage of this feature:
When enabled, users see primary record details immediately. As the related list data loads, users see a progress indicator. Separate loading can improve performance on record detail pages for org with large numbers of related lists. This option applies only to Salesforce Classic and is disabled by default. The options for separately loading related lists don’t apply to Visual force pages, the Self-Service portal, or other pages for which you can’t control the layout.
This is the blog to Improve the performance of an application and follow the steps to do lightning as well as Classic.
Step–1: Open the Setup in the below image
Step-2: In setup, Search for the User Interface and click it.
Step–3: Open the User Interface option, Once opened we can see the below
Step-4 Select the below option as selected other options.
Step–5 : Enable the option ‘enable separate loading of related lists’.
Finally, we can use this feature when we want to improve the application performance
For more Blogs : How to Turn Off Salesforce Classic
For Reference : Click here