Thing to help me remember.
Rendering for Interiors and Exteriors. Exteriors are way easier to render as there are far fewer bounces.
Primary Bounces need an accurate calculation system whereas secondary requires something less accurate and faster.
Irradiance map is the most efficient speed/ accuracy making it the obvious choice for primary bounce especially for interiors.
Light cache is a good secondary as it is not accurate but faster.
For the GI, we used Irradiance map for the speed and efficiency but when it comes to the animated objects, we switch to brute force since it doesn't flicker and the most accurate in results. This is the same reason why we don't use light cache for primary bounce in the flicker free post posted earlier.