Do invisible polygons influence rendering time using Cycles renderer? TL;DR The number of polygons has negligible impact on rendering time.
I'm having a furniture designed part by part using FreeCAD and exported as Wavefront OBJ (following Tutorial Render with Blender). This furniture is then placed in a very simple approximation of my apartment (also modeled using FreeCAD).
The problem is with rendering time. With 8K resolution and 2048 samples this is ~14 hours on (not built for this purpose) entry level 8 core 2nd generation Xeon Scalable Processor. With this amount of samples noise is still clearly visible, especially in shadows. Going down to Full HD reduces time to some 50 min.
This amount of time is in odds with what I found on the internet, where (apparently) much more rich models tend to render in an order of minutes.
This makes me think about one thing: my furniture model is probably much more detailed than average model of object used for rendering in blender. In particular there is plenty of polygons inside the object which are clearly not visible. Do these still slow down the rendering?
If yes, is there some tool which could automatically simplify FreeCAD models for rendering removing invisible polygons?
Just for the record: I don't have any GPU capable of rendering.
EDIT:
I made one more super simple experiment: I removed all the furniture leaving just an empty apartment, light and camera. It had negligible impact on total rendering time (Full HD, 32 samples). In retrospect I could have done more to answer this question myself.
Blender calculates all geometry in the scene at render time.
Here is a very simple example:
Suzanne alone:

render time 00:12:30

Suzanne with torus and cube outside the camera field of view:

render time 00:14:37

The scene from the above example:

The difference in render time is not alot here, but in a large scene could become exponential. If the object is redundant then remove it. Interior geometry that the camera will never see can be in many cases safely deleted. This will only improve render time.