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Because enthusiasts who think they are so awesomely clever looooove disabling anything that makes the computer look pretty, because they're convinced that effort making the computer look pretty is clearly done at the expense of performance.
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It's a little unfair to call out on those of us who believed this actually improves performance, as the button is clearly called "adjust for best performance." In any case, I like the Windows Classic theme, which disables composition altogether. So these were wasted 5 minutes to try and restore that setting to "best appearance."