With the move to gcloud - I'd expect the time for dynamic content to speed up a LOT.
regarding 15 seconds currently - that seems astonishingly slow - so I'd check with the implementation, it should be fairly quick to load content.
In terms of SEO - google's coming across a lot of personalised content, with sitecore, hubspot etc all doing this - so i wouldnt say it will aversley affect SEO unless you have the meaning of the page very different according to the marketo personalisation. For example, if its a "what we do" page and you've just allowed for content focused on their industry, then the essential meaning and purpose of the page remains the same.
hope that helps!