It is impossible to say without seeing images, or preferably the scene. For example, it is best if all lights are fairly similar in intensity if you are going to use LightMix (and you then set final intensities in the LightMix itself afterward), because if one light is extra bright, then due to adaptive sampling Corona will assign more importance to that light and less to the others. Then if you turn that extra bright light off in LightMix (and usually this would be a Sun), you might find there is noise in the parts of the scene that were illuminated by that light, because the other lights reaching that point in the scene were given less priority. Maybe that is something that is happening here.... or maybe not :) Without seeing some images, can't really say. You also have to be sure that denoising is turned on for every LightSelect, and you have to be sure to be using a recent version of Corona (earlier versions, from the early days of LightMix, could not denoise the "Rest/Unassigned" layer for instance), but other than that those kinds of situations, there is no reason why LightMix should not be as noise free as rendering without it.