Computing Interior Absorption Factors - what does it do?

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Hi,

I'm running a DesignBuilder simulation, where I've tried simplifying by merging rooms together by identical activities. The simulation screen is stuck at "Computing Interior Absorption Factors" and I fail to understand why it is taking so long, or what it does exactly. I've tried searching the Engineering Reference for EnergyPlus and the forum here, but no luck so far.

Can anyone explain to me what is happening at this stage?

Thanks!