So, on a test site I just moved my
Of the two benefits (progressive rendering and better download parallelization) the progressive rendering makes a big differences for certain pages. For example, the admin/build/menu page problem is reduced - users see the page content and then the interface freezes while the JS loads/modifies the page. This makes it seem like "oh, js is working" as opposed to "woah, white screen of death!"
They mention that not all scripts can support this, such as document.write. AFAIK we don't use that much (at all?) so I feel like this is a good improvement. Has anyone else done this? Any advice you can share? Any reason not to do this for all themes in Drupal7?