Available with Network Analyst license.
The network dataset layer is rendering dirty areas. Normally, a web client does not need to see dirty areas, and rendering them takes longer than not.
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 network dataset layer, click the Symbology tab, then uncheck Dirty Areas.
Polygons representing the area of a network that has been edited since the last build process are drawn when dirty areas are turned on. This information may only be useful to network editing applications and can generally be turned off for normal applications.