Rays per sample too high
There are several measures of rendering performance other than “how long do I need to wait for the noise to go away”, that can be used for performance debugging, (i.e. determining why a scene is taking too long to render). One of them is "rays per sample" displayed in the render stamp and in the “Stats” panel of the Corona VFB.
Rays per sample
Each image sample involves casting a single primary ray from the camera into the scene. This ray can then split/generate many secondary rays. The number of generated rays depends on the scene and influences performance - more rays/sample means a lower number of passes can be rendered in a given time. The number is also directly dependent on the GI vs. AA balance and LSM - higher values will result in higher rays/sample. This is normal behavior and not a bug. The usual value of rays/sample is about 5-50 for GI vs AA samples set to 16 and light samples multiplier 1. Using UHD Cache (3ds Max | C4D) results in lower rays/sample and better performance over time.
Very high rays/sample for given GIvsAA balance means the rendering is doing too much work for a single image sample, usually because of an incorrectly set up scene, decreasing the overall performance. Common reasons for high rays/sample include:
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Too high albedo of walls in an interior scene. Albedo is the percentage of energy a material reflects, and in Corona is the sum of diffuse, reflective, refractive, and translucent components. In reality, almost no materials have near-white albedos, and using white albedo (for example 255-white diffuse color) in rendering gives very unrealistic and slow results and cannot be correctly rendered with an unbiased renderer. Try keeping albedos of all your main scene objects under RGB 180, and get whiter walls by brightening light sources.
See: What is albedo? -
Excessive use of transparent/refractive materials
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Complex blend/layered material setups
To solve the problem, try: