There's no reason to test a GeForce GTX 1080 Ti at 1920x1080; at that resolution, even our high-end host processor bottlenecks performance. Rather, these cards are designed for 2560x1440 and up.
At QHD, the ROG Poseidon GeForce GTX 1080 Ti 11GB Platinum Edition is, depending on the game, clearly faster than the GeForce GTX 1080 Ti Founders Edition and Titan X (Pascal).
At this high resolution, Asus' card is again quite a bit faster than the GeForce GTX 1080 Ti Founders Edition and Titan X (Pascal).
Many of these titles are playable at 4K. Those that aren't can be made smoother by dialing down the quality settings somewhat.
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