Available with Network Analyst license.
The service area layer is set to trim output polygons. This can take longer to compute.
Solutions
Take any one of the following actions to resolve this issue:
Do nothing. Mark this warning as an exception to avoid seeing it again.
Automatically correct this issue by choosing the appropriate option on the warning's context menu.
Open the properties of the service area layer and uncheck the Trim option on the Polygon Generation tab.
More information
The service area polygon boundary can extend significantly in areas with sparse road networks. The Trim option can reduce the outer polygon boundaries to make them more compact and possibly more pleasing visually. If web service performance is a concern, publish the service area solver without using the Trim option.