While, ""insert_dns_prefetch" filter inserts <link rel="dns-prefetch"> tags to allow the browser to pre-resolve DNS for resources on the page," google added.
In addition, mod_pagespeed 1.2 can proxy and optimize resources from trusted domains. "This feature enables you to optimize resources even from servers that don't run mod_pagespeed. Beyond compressing and cache-extending such resources, this can improve performance of sites running SPDY where the best practices for performance are to serve all resources from the same domain (see mod_spdy)," google adds.
Additionally, convert_jpeg_to_progressive is now a 'core' filter." "Large JPEG images are now transcoded to progressive." Both of these improves the browser experience and makes such files smaller.