This happens occasionally, and seems to rear its ugly head unexpectedly and repeatedly when I am on certain projects. The render will start, completing the pre-pass smoothly, then at 50% or 66% through the final pass it freezes for about 15 minutes then resumes and completes without a hitch. All the while during the interruption, the render clock is ticking away while no progress shows on screen. So in the background something is going on, but what?
I know that there are certain factors such as path tracing, reflections, water, or displacements can slow down a render and I always take those under consideration. What seems to be associated with the freeze is the convergence between sky and terrain, though I don't know why it would.
I am using a renderer with customized sampling settings, one that Oshyan set up for me when I was experiencing a very slow render time on a certain project. When I switch to the legacy sampler, the same thing happens so it's not the settings.
Has anyone else experienced this?