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Windows 11 scores dead last in gaming performance tests against 3 Linux gaming distros
(www.notebookcheck.net)
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I interpret it as the time taken to render a frame. Unlike FPS which is basically a moving average (or rather 1 divided by the average frame time), frame time is a single data point. Collecting frame times allows you to do things like compute the median or, in this case, the lowest 1% of the frame times. That can give you a better idea of how smooth performance appears to the player, and what the worst-case performance is like.