To speed up my website (or at least the time to first byte (TTFB)), I'd like to only send the layout of the page and then get the page content with an asynchronous request. This way, the TTFB will be really fast, because this will be short, lightweight and cached content. For the content to be indexed, I will analyze the User-Agent to know if I have to send the content the old-fashion way.
Is this a safe way to do so, or will I be blacklisted by Google (because of the different content)?
Thank in advance !
PS.: I know about the Google AJAX craw method (https://developers.google.com/webmasters/ajax-crawling/), but this is not really what I want.