Thanks for advice yes I already placed portal light (plane object) near the entrance. Documentation for the portal lights is very interesting I didn't know how placement of portal light can impact the scene. Is there a way to put down render time even more? Curently I have in scene one sun light and one portal plane light near the entrance of the cave, so far render times are about 6 hours on Ryzen 2600x.
For the scene I worked on fixing geometry for the cave scene, but leaving similar material setup. I hope render will be finished in few hours :)