i would recommend using gtmetrix on all the demo sites and your own with GDF. i use visual composer at the moment with GDF and it is still below 800kb for the front page while avada,x were well above in size and loading time. it is absolutely fine for low traffic but when you have active visitors on the site with a premium shared host still it will take its toll.
i would honestly love to see a child theme that has a really nice header and all the compatibility with bbpress,buddypress,woocommerce AND visual composer.
with this we could build any kind of look for the site with keeping users online too and even sell some stuff 😉
ps.:i tried avada,x,kleo,GDF,Modern childtheme and now GoodNews too(still in progress to make it compatible) right now im on modern with some css mods thanks to the support from GD 😉 my whole homepage is built in VC using the widgetised sidebar and other elements. just bought another extension where the programer will have a look to add the GD shortcodes too to have them available as VC element 😀