Is it be better for performance to make my .png transparent or add the transparency within construct or is there no difference?
I don't think it would make a lot of different, unless you mean that you want to use some filters etc within C2 to do it. But doing it with PNG is a hell of a lot easier I think, as it is part of the PNG format by default.
thanks again nimos, agreed much easier
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It will have exactly zero impact on performance. A basically complete list of the things you should worry about performance-wise is here: https://www.scirra.com/manual/134/performance-tips