Available with Network Analyst license.
The network dataset layer is rendering traffic. Normally, a web client does not need to see traffic data, and rendering it takes longer than not.
Solutions
Take any one of the following actions to resolve this issue:
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Do nothing. You can ignore this message if you are publishing a service to display traffic conditions.
Automatically correct this issue by choosing the appropriate option on the warning's context menu.
Open the properties of the network dataset layer, click the Symbology tab, then uncheck the traffic source.
More information
A network dataset can have historical and live travel speeds built into it. If the web client is not using this information, displaying the data causes excess data-transfer and rendering times.