Google’s Earth team says it’s improved memory utilisation to show more buildings and other content when you’re zoomed in, and worked on reducing “stuttering”, or frame drops, to make the flying feeling more smooth and, well, fun. Google goes into detail at their blog post, but we can verify that, having last used Earth in its 4.0 release, 5.1 definitely feels a bit more fluid while grabbing and pulling past our neck of the woods.
Google Earth 5.1 is a free download for Windows, Mac and Linux systems. A browser plug-in auto-installs with the Windows release, and Windows versions also load the Google Updater; the Google Operating System Blog points out a direct Windows installer link to avoid the Updater.
Google Earth 5.1: Think fast [Google LatLong Blog]