Available with Network Analyst license.
The service area layer has line generation enabled. Computing and returning many lines to the client can take longer than doing so for a few polygons. If the goal is to use this service for polygons, line generation should be turned off.
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 layer properties of the service area layer, click the Line Generation tab, then uncheck Generate Lines.
A service area solver is normally used to compute polygons. The actual lines traversed may not be needed by the client application. Also the number of lines that make up a service area can dramatically increase for larger break values. Turn off line generation to improve performance if the client application does not need this information.