Our MapGuide map and imagery performance is largely due to:
Of the three, the fast servers are probably the least important.
The source imagery files on the MapGuide server are tiled and MapGuide uses a "Raster Image Catalog" to identify just those files needed for the user's request.
The MapGuide server resamples the imagery on the fly to something appropriate for the user's viewing window and scale, reducing transmission volume and time.
The layers on a MapGuide map are processed and served independently, allowing new layers to be viewed without regenerating and sending other layers. (For instance, turning on the orthophotos does not require regenerating or transmitting existing vector street segments in the user's current view.)
Further, for most imagery, we keep the original and most detailed imagery files (typically 1 foot resolution), as well as subsampled files at resolutions of 2, 4, 8, 16, 32 and 64 feet. The MapGuide Viewer knows the user's current viewing scale and requests a subsampled resolution appropriate for the current viewing scale. You can see the resolution currently being used in the map legend on the left.
Note that keeping these subsamples on the server is in addition to MapGuide's dynamic resampling on the fly for the current request. The subsamples on the server reduces the server I/O and processing load, while the dynamic on-the-fly resample by the MapGuide server software optimizes it further for the user's current view.
With the exception of keeping the subsamples on the server and designing the map to use them at the appropriate time, the rest is all built into MapGuide and works transparently.