my question was just because ‘small illuminants’ is for lighting predominantly by small lights and yours is by a luminant texture so the setting you are using might not be the one you are looking for.
about the more general questions, no you can’t prevent flicker and splotches without increasing render times, you have to have render power but there is lot you can do with the settings, if you have not done so check out the help section ‘Tips and Tricks (Global Illumination)’.
Don’t get stuck on Physical Render ‘just because’ sometimes Standard is faster (and vice versa) – test which one is faster for the scene you are doing (unless you need something only one or the other will do.