“With portal, the image is brighter in shade areas.”
You hit the head of nail and totally didn’t know it was there. To reproduce the brighter shadows you need more indirect bounces and/or light tricks. An often times the light tricks cause more noise which means you need more samples or deal with more blurring in denoising. Portals are more about realistic lighting of indoors from outdoor lighting coming in thru an opening(AKA Portal) like a window. Also, as Omar points out in an animation even fractions of a second add up. Take for example 1 minute of animation. If you’re using 24 FPS that is 1,440 frames. If you use 30 FPS that’s 1,800 frames. Plus as the camera moves this will effect render times as well. As Martin points out, it very much becomes a balancing act of quality vs speed.
