Very interesting question and something probably not many people think about. I don't know if this would work but have you tried adding the form to a popup window instead and just have a button on the landing page. The page will still need to load the marketo files but it might not need to fetch and load them before the above-the-fold content has loaded.
That supposes that the form is not itself above-the-fold content, which is sometimes true (if it appears only in a footer) but usually not.
My view is using an automated tool without understanding the technical meaning of each point just leads to unnecessary alarm. "It is drop all our site ranking" -- is there proof of this or just a guess?