I found a wiki reference that discussed marker optimization techniques and tried some of them to see the differences. There are others besides these, so if you are interested visit the link.
So, what to do? I am considering trying a combination of the approaches above as well as delaying the loading of the markers. For example, at the highest zoom level, all markers are visible, so I might wait about half of a second before adding markers. Then when I’m adding, I allow about an eighth of a second between batches, where a batch is 20-25 locations. Instead of using the GMarkerManager at the highest zoom level I would just use addOverlay. This gives the map the appearance of pretty smoothly adding markers to the map. Although it is kinda cool to watch, adding one marker at a time takes too long, so it has to be a few at a time. Not so many that it appears chunky though.
The other techniques that I think might be useful are delaying the InfoWindow data until the onclick event as well as factoring in the viewport area for marker display. Theoretically, this should allow me to load all of the markers with the smallest amount of information possible and increase the performance as the user zooms in.
I really don’t like the clustering approach as is tends to obscure the data. Part of what I am trying to accomplish with using Google Maps is to present my data in a way that allows for easy visualization of the points in relation to each other. Clustering seems to defeat that purpose. I also like the shadows because it makes it look nicer. If it comes down to it I will, reluctantly, lose the shadows.
My other options are to disallow viewing all of the locations at once. This might be accomplished by loading an empty map and asking the user to search first. Additionally, I could limit the search results to a few hundred. I’m not keen on this approach, but I may be forced to go this route if I can’t improve the performance otherwise. I could also try using an XML file with GDownloadURL, but I can’t see that actually being faster.
I had an extremely hard time finding information about Google Maps and performance, including tips, techniques, hacks, etc. If you have any suggestions please post in the comments. It would be very interesting to see how other people are handling this problem.