The data source for this layer does not contain a spatial index, which will affect drawing performance.
Build a spatial index for your layer's data source. See the Add Spatial Index geoprocessing tool for more information.
The spatial index is used to quickly locate features when you display, edit, or query data. Therefore, an appropriate spatial index is important, especially when you're working with large datasets.
It is very important to build a spatial index for your datasets. This will significantly increase display performance as you navigate around your map.