Google has released a new version of Google Earth - version 5.1 (beta). This version is mostly dedicated to improving performance. They have improved memory utilization, graphics performance, and even bandwidth related enhancements. They also reduced the start-up time by 25% in most cases. For the Windows version they have included the Google Earth plugin as part of the installation so most users will automatically have the plugin ready when they visit a web site requiring the plugin.
Another thing Google doesn't mention in their post today, is that I'm sure they have addressed many other issues since the release of 5.0 back in early February to help more people have a pleasant experience with the application. So, try downloading Google Earth today for a faster and, hopefully, even more reliable experience! But, notice that there is a "beta" attached to this release. Google normally releases a beta version first - then if there are issues with the release they will release a final (non-beta) version fixing as many issues as possible.