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

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?

Software: OS X 10.8 - Poser Pro 2012 SR2 - Luxrender 1.0RC3 - Pose2Lux
Hardware: iMac - 3.06 GHz Core2Duo - 12 GB RAM - ATI Radeon HD 4670 - 256 MB