This plugin does not cache widgets automatically! You must define which widgets get cached.
Add a widget to the cache and designate which actions the cache gets flushed on.
$widget_cache = Voce_Widget_Cache::GetInstance();
$widget_cache->cache_widget( 'Archive_Links_Widget', array( 'save_post' ) );
$widget_cache->cache_widget( 'WP_Widget_Recent_Posts' );