Ussually that is Global Illumination, which, unfortunately, generally needs to be high quality for interior renders and is the main culprit for long render times.
There will be 'spots' too, most likely, which are not as noticeable in a still frame, however once you have an animation sequence they will get very noticeable.
Assuming the memory leak that used to plague Cinerender in AC18/19 has been fixed (I believe it was), your best bet is to render overnight. I recommend rendering to images like PNG and assembling them in something like Adobe Premiere to make the video clip. That way if the render crashes somewhere along the way, you will have all the frames it rendered up to that point and just need to render whatever is left.
If you don't mind a slight step down in quality, consider that BIMx offers you ways to export fly through, limit navigation to still cameras and adds the benefit of doing the cool 'show 2d section / plan in 3d' trick and generally takes like 20 mins to prepare with GI calculation ticked.
Erwin Edel, Project Lead, Leloup Architecten
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