In Macintosh Manager Admin, under the Server Tab, there is a graph that represents AFP File Server Usage of the Macintosh Management Authentication Server. The purple areas represent the actual server utilization for that time and the black line represents the average usage during that time. (See Figure 1)
Figure 1 AFP File Server Usage graph
The black line is usually most important when reading this graph to determine the sustained server load. It's possible (for example) to have a peak for a second or two and the purple will hit 100 percent but the black line might show the average usage to be around 20 percent. If the black line consistently runs high (above 80 percent), you should think about splitting up your Documents Storage between multiple servers or upgrading your current Macintosh Management Server to a faster server.
For information on saving files between multiple servers, see article 31265: "Macintosh Manager 1.3: Saving Workgroup Documents to Another Volume."