This plugin caching all js file to local cache directory, you may also apply gzip or deflate to this folder, using your webserver.
It also minify response time for each external JS files because no more need to DNS request it, establishing connection, receiving data from another server. Your server process such files in one keepalive connection. Not forget to chmod /cache/ directory for writing and enable the plugin.
Download JS Cacher 1.0 for Joomla 2.5, 1.7 (1.6 too) plugin