I've discovered that using a hidden class secretly enables asynchronous mode. I think that should be flagged up explicitly in the settings page.
Also, I've discovered that the non-async mode doesn't work when the link contains an image:
(the large image is linked to a PDF, and the page doesn't render right with the mime icon, perhaps because the CSS sets the height to 16px?)
But my main problem is that the async JS is causing a clash with another dynamic element on my page here:
The [show search options] button will not work properly when async replacement is turned on. I'm not good enough with JS or Jquery to be able to analyse the problem. Can you help please?