For me, the go-to game is The Witcher 3. Any time I invest in new hardware, that's the game I load up to check the performance impact. But really, it should be Warner's grotesque PC port of Arkham Knight, shouldn't it? Any machine that can run Arkham Knight at speeds in excess of 'flip book animation' ought to be granted an actual knighthood. This video then, demonstrating a GTX 1080's 4K performance of Batman and GTA V in 4K, makes for an intesting watch.
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YouTube channel Joker Productions tested a GTX 1080 Founders Edition with an Intel Core i7 5820K and 16GB of RAM (full specs list here). In the video below, you'll see how it performed at low, medium, high and ultra settings in 4K. Take it away, GTA V and Batman: Arkham Knight:
So, what did we just learn? Well, it appears we learned that the GTX 1080 isn't a 4K card. It does manage to exceed 60 fps at times in both games on ultra settings, but in neither case achieves an average, stable 60. And we're talking about two games from the earlier half of 2015 here. Admittedly games that are highly scaleable (GTA V) or notoriously badly optimised for PC (Batters).
Both games are DX11, too. It may be that DX12 will help performance at 4K in the titles its implemented in, but what often happens as an API beds in is that the potential performance increase is offset by a developer's (quite reasonable) desire to lift the fidelity ceiling.
Then there's the system to consider. No, it isn't absolutely state-of-the-art. A Skylake system with an overclocked 6700K will buy a few more frames, but would it get the frame rate up to 60?
One last thing to consider: overclocking. Joker isn't running the GTX 1080 at overclocked settings, so there's scope for higher frame rates by tinkering. But we're talking about a graphics card that released less than two weeks ago at the time of writing.
It's a conflicting experience to watch that video. On one hand, it's very impressive to see a single card do what two high-end cards would have done last generation, but I think it also falls slightly short of what people might expect of an $833(!) Founder's Edition card that has been hyped as an SLI 980 ti killer.