in the right corner, where your used ram is displayed -> RMB
BUT, i think this is not what you looking for.
to your question: if u have a loader in the foreground of a merge, the rendering will go on, if it is missing.
But, if the loader is the background at all, then the rendering stops.
how to avoid? try not to use missing pics
or merge them over a black background-node wit alpha 0.