After playing with this for a couple of hours, here is my current configuration:
pagespeed EnableFilters defer_javascript,collapse_whitespace,combine_css,combine_javascript,elide_attributes,extend_cache,remove_comments,insert_dns_prefetch,lazyload_images,inline_javascript,local_storage_cache,outline_css,outline_javascript,prioritize_critical_css,rewrite_css,rewrite_javascript,rewrite_style_attributes,trim_urls;
The option that gives the best result is defer_javascript. Especially on our forums, where we have several advertising banners running, this made the page load first, and then the banners. Fantastic results. However, this option generates a conflict with the >Lazyloader hook, and the images are shown as broken The solution was very easy: I disabled this hook and enabled the integrated lazy loader (lazyload_images).
Additional comments and suggestions are more than welcome.