Even before setting up memcache and opcache (and installing any add-ons), users, including myself, were/are running into an issue from time to time where an attachment image fails to load (or only partially loads) when a thumbnail is clicked. Right-clicking on the thumbnail to open the image in a new tab (Firefox 41.0.2, but also in other browsers it seems) will sometimes eventually load the resource and other times just get the "Connecting..." tab name with the "spinning" GIF and never deliver the resource.
Since it seems to be across browsers, I'm guessing it is a server configuration issue. Any suggestions on what to do? Any magic to work with php.ini? Or maybe $config['enableContentLength'] is an issue?