Quote - Understanding how to set up portals is going to be good in general of course.

While you're right of course, I think it's a minor feature, as it only serves to accelerate a very specific type of render (interiors with windows to a "daylight" illuminated exterior).

Quote - But I was also thinking that maybe the interior of the warehouse might be causing a lot of extra bounce calculations and creating a portal where the door is could then maybe improve efficiency?

I don't think this should make any visible difference. Not any more than the barrels...

The problem isn't bounce calculations, but the ray going for the camera looking for a light source (we're rendering bidirectional): It might make a big difference if 80% of the scene is a light proof wall, but the 5% the door represents in this case shouldn't make any big difference, I think.