Available with Network Analyst license.
The network dataset is being drawn with system junctions. This dramatically increases the amount of data that is sent to a client.
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 layer properties of the network dataset layer, click the Symbology tab, then uncheck System Junctions.
Open the layer properties of the network dataset layer, click the General tab, then set a scale range on the layer to limit the drawing of the system junctions.
More information
Generally, the network layer should not be rendered on the client unless the application needs to see some particular network attribute, such as historical travel times. You should try to render the network dataset's source feature classes, such as streets, rather than the network dataset layer itself.