Does this new feature remain on at all times during final rendering to improve rendering speed?
Does it have any side effects?
If the number of samples is low, it will not have much effect?
The idea is that denoising takes considerable time on each sampling pass - from several up to dozens of milliseconds depending on resolution. This option lets you only denoise the image that will be saved to file, speeding up the whole process. It should not change the result you get. The only downside is that you will see a noisy image on the monitor while rendering. No other side effects are expected. If there are, there may be a bug.
As I expected.
Don’t Denoising Frequently During Rendering