A tracking layer in your map is referencing a very large dataset, and this could cause poor drawing performance.
Analyze the drawing performance of your tracking layer in the Preview ArcGIS Server window. If the performance is unacceptable, consider reducing the size of the dataset. If you cannot reduce the size of the dataset, try to simplify the symbology used by the tracking layer. For example, removing labels and using simple symbols can have a significant impact on drawing performance.
Generally, tracking layers are engineered to have fast drawing performance, especially when changing the time extent of your map. However, if a tracking layer is referencing a very large dataset, the drawing performance can still become poor.