Does it also have this frame rate issue during the day? Very slow shutter speeds can cause frame rate to be reduced, but I would not expect that to happen only on the main stream while the sub streams achieves the full 15 FPS....
I notice the bit rate that
Blue Iris was receiving is less than half of what the camera is configured for (8192 Kbps in camera should be about 1024 kB/s in Blue Iris after unit conversion). Since you have Variable bit rate quality set to Highest, I would expect the bit rate to be higher. Is there any wifi involved to get the video stream between your camera and Blue Iris?