OK, well. This thread has gone in an odd direction, but I’m seeing something similar.
I thought I was having this same issue with my outdoor scene and Shadow Catcher. If I rendered in the VFB or the C4D Picture Viewer, the first frame would take 13:30 sec to even show any part of the image. After that it went quickly. This same scene with just hitting render in the viewport took :12 sec. Huge different when working.
My solution was to restart C4D which worked once. Now it does it all the time. Any other ideas?
EDIT OK so I found what was causing this slow down for me. Recently, I discovered that an old plug-in for C4D was updated for at least r17. It’s called Roll-It, a very hand Python script that used to use for simple car tire rotations. Simply apply the tag to each wheel and move the parent object. Roll-It rolls the wheels at the proper speed. Super simple, BUT I couldn’t get real-time feedback and that 15 min render time even with the tags disabled. I just deleted the tags and boom, rendering back to normal. Not sure if this is an issue with Corona or the plugin, but thought I should mention it anyway. Maybe there could be issues with other Python scripts and Corona. Here’s the link: