A layer in your document uses representations with symbol level drawing and feature masking enabled. The use of symbol level drawing with masking can affect display performance in map services.
First, check the drawing performance of your map service in the Preview window. If you see the need to make changes, do one of the following.
Map services use a graphics pipeline that can employ a high-performance processing method to draw symbol level drawing. However, feature masking is incompatible with this high-performance method. If your application warrants the use of feature masking in conjunction with symbol level drawing, you can choose to mark this warning as an exception to avoid receiving the warning next time you analyze the map. The map service will use a slower method to perform symbol level drawing in this case.