I have a website where we use a technique to speed up loading times based on a combination of AJAX + caching. Basically, when we have a section in a page with content which is slow to retrieve, we first check if it's cached. If it is, then we serve the content; if it's not, we serve a placeholder and then make an AJAX call in the client to retrieve the content, which is then cached for subsequent requests.
As a consequence, sometimes you get the entire page content in the first request, and sometimes you get those placeholders, which get filled immediately with the responses of the AJAX request.
You can see an example in the results count by category in the right column of this page:
I'm worried if it could be seen as cloaking by search engines because if you make a request for a page which content isn't cached and then ask again for the same page, you would get different responses, the first with the placeholders and AJAX requests and the second one with all the content rendered.