Hi @maxrezn, thanks for the question. Normally, amount of images and page size is the main factor, but there can be others too… I will go check out your site and run some tests from my perspective, and get back to you with some options to check out.
Since this content is loading into an iframe, it should behave asynchronously – meaning it will not block the rest of the page from loading. Ideally, GT Metrix should take this into account… but it looks like they simply wait everything to load (including iframes).
However, if you prefer to manually insert the youtube video (via custom code) as to help your benchmark results, we can certainly advise a way to do that.