Unfortunately in Blender it is very expensive to update the rendered image during the final render.
I fear we can't change that. We can investigate again of course, but I doubt we can do anything about it.
So the value has this minimum to prevent users from spending a lot of CPU time on framebuffer updates. It is probably even too low.
Very noisy renders
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Please upload a testscene that allows developers to reproduce the problem, and attach some images.
Please upload a testscene that allows developers to reproduce the problem, and attach some images.