D: Controls that require extra data beyond the primary record produce the most strain on form responsiveness and loading speed. These controls fetch data over the network and often involve a waiting period (seen as progress indicators) because it can take time to transmit the data.
Keep only the most frequently used of these controls on the default tab.
Incorrect Answers:
B: Use asynchronous network requests when requesting data. Request data asynchronously rather than synchronously when extra data is necessary for customizations.
C: Moving scripts to the Onload event would slow down the loading of the form.
Reference:
https://docs.microsoft.com/en-us/powerapps/maker/model-driven-apps/design-performant-forms