A low-quality HDR can produce bad shadows and noise. Also, if your system is running out of RAM it will swap with the hard drive which is dramatically slower unless you have super fast SSD. Also consider the resolution of your materials. In my opinion, only the trees near the camera could use such high resolution textures. It's just overkill unless you are rendering different shots in the scene then that makes sense. C4D loads all of those 4k and 6k images into RAM. I don't know the sizes you have, but I can imaging each of them could be hundreds of MB or more for each one. That CPU is not world-shattering speed-wise. I see online it's does 12195 in the Cinebench r23 multi-core test. For example, my 5-year old iMac Pro comes in around 16737 in multi-core. Either one is nothing compared to Threadripper 64, but then again, that $4,000 USD just for the CPU. I get it.
What I love about Corona is you can still get decent speed on older CPUs. I think it's a matter of learning to adjust your scene for best optimization.
Do a quick test. Change your sky to Physical Sky (no HDR). Maybe add a Sun to match your shadows. Set your settings to match mine to override all materials and render a test. Try 100 passes or use the Noise Level Limit to 5 and hit render. See if you still have noise. If you do NOT, then I suspect it's the HDR creating the noise.
Sorry, I'm rambling a bit. Post back your results. Good luck.
Another thought. If it really is such a small area giving you trouble, consider rendering just that area with more passes, then just paste them together.