If you import a large PNG with no alpha you will have slower frame rates
than importing the same images as a JPG even at 100% compression,
regardless of the Flash export settings for that image. You could set
the PNG to export as an 80% JPG and it will still perform worse than if
you imported a 100% JPG to begin with.
The hows and whys are interesting to know, but irrelevant. Your
animations will slow down if Flash has to deal with a larger source
file. I've done my own testing and have proven it to be true. Since I
can't depend on the end user having a great video card, it's more
important to maximize performance across the board by importing 100%
JPGs when transparent PNGs are not necessary.
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