I've been playing both with your plugin and with the raw functionality of the GDE and found more and more that the response time of their service is sketchy at best and often completely fails.
I've been thinking that it would be great if the plugin could support the option to cache the images and XML returned from the first call to GDE so that following calls would return the local data without Google re-parsing the document. Basically, after an initial call to the Google API, all future calls would be handled on the user's server. XML, page images, and page count could be stored as post meta values.
In the project I've been developing, I've already added functionality for retrieving and storing the the XML document and retrieving images should require the same basic process.
Obviously this functionality would require retrieving & storing the images as well as the XML. Frontend, it would require a rewrite of the JS to retrieve from local files for any cached pages.
If this is functionality you would be interested in having in the plugin, I would be potentially interested in assisting.
All the best!