Does compression on export have any effect on gameplay or just download/bandwidth as the manual/blog says?
Doesn't affect game performance (framerate etc.)
Download only - images are usually decompressed to bitmap when starting the actual game, regardless of whether they were sent as PNG or JPEG.
does semi transparency in the image affect performance?
Awakening - no. Nothing affects image performance at runtime, other than if you draw lots of images, or use blend modes or shaders.
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So if I suffer through the brute compression with each upload then it would have a net positive on playability through smaller download size with no adverse affects outside of watching the paint dry while it compresses on export?
Sounds about right, but brute only tends to get you 1-2% more compression while taking 3-4x longer, so you might as well choose standard!