There’s one problem with Gravatars. They are very slow to load. Each page with comments on them makes one call per comment to the Gravatar server. While a single call takes only a 100ms, on a page with hundreds of comments, we are talking about major slowdowns. Page loads can take 20 seconds and more.
This plugin solves this problem by:
caching gravatars with WordPress cron job..
caching gravatars on comment submission
maintaining a single copy of the default gravatar instead of downloading it again and again for all the email addresses with no gravatar associtated
You can use the built in installer and upgrader, or you can install the plugin
After installing, make sure you visit the plugin settings to make sure the plugin works correctly. You will see a check if the cache directory is writable and you will have to set the gravatar size (plugin also uses autodetection mechanism, which might come handy).