Check out Google’s new Marker Manager for the Google Maps API (Examles here). It helps improving performance and usability for maps with many markers; read for yourself:
"Adding a large number of markers to a Google map may both slow down rendering of the map and introduce too much visual clutter, especially at certain zoom levels. The marker manager utility provides a solution to both of these issues, allowing efficient display of hundreds of markers on the same map and the ability to specify at which zoom levels markers should appear.The marker manager offloads management of markers registered with the utility, keeping track of which markers are visible at certain zoom levels within the current view, and passing only these markers to the map for drawing purposes. The manager monitors the map's current viewport and zoom level, dynamically adding or removing markers from the map as they become active. In addition, by allowing markers to specify the zoom levels at which they display themselves, developers can implement marker clustering. Such management can greatly speed up map rendering and reduce visual clutter."
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