Magento/PageSpeed

https://developers.google.com/speed/pagespeed/insights/

http://www.webpagetest.org

in app/design/frontend/base/default/layout/

et_advancedcompare.xml
googleanalyticsplus.xml
page.xml

add <params>async</params> to addJS

WARNING Do NOT add 'async' to

<action method="addJs"><script>prototype/prototype.js</script></action>
...
<action method="addJs"><script>prototype/validation.js</script></action>
..
<action method="addJs"><script>varien/js.js</script></action>
<action method="addJs"><script>varien/form.js</script></action>

or add to cart will not work anymore (TypeError: productAddToCartForm is undefined)

see http://stackoverflow.com/questions/28658063/html-script-async-attribute-in-magento