Available with Network Analyst license.
The service area layer is set to compute polygons with the detailed option. This can take longer to compute.
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.
Alternatively, open the properties of the service area layer and, on the Polygon Generation tab, click Generalized.
The detailed polygon option can be useful in cases where the outline of the polygon needs to be highly accurate or the interior roads that couldn't be reached need to be accounted for. For the majority of web applications, the generalized approach is adequate and much faster to compute. Changing the service area to Generalized rather than Detailed will result in better performance and produce fewer (or no) holes.