Offloading a portion of each frame's rendering tasks to a secondary GPU can not only speed up frame rates, it can also help create a smoother gaming experience overall, as evidenced by Civilization: Beyond Earth's use of "split frame rendering" in CrossFire setups with AMD's Mantle API. (Traditional "Alternate Frame Rendering" for multi-card setups has each GPU alternate rendering a full frame--hence the name.)
Explicit Multiadapter sounds like PC gaming's Holy Grail, but it remains to be seen how widespread support for this fantastic feature extends, as developers will need do a lot of grunt work to make it work. PC Perspective has an excellent breakdown of this complex topic.
This sounds pretty familiar
You're right: DirectX 12 sounds a lot like AMD's now-defunct Mantle API for Radeon graphics processors. Mantle did all of this long before Windows 10 was announced. After its release, AMD executives mused that Microsoft would likely never release DirectX 12, then bam! Microsoft announced DirectX 12, which basically apes all of Mantle's key features on a far wider range of hardware.
Will DirectX 12 actually make that big of a performance difference in my games?
All signs point to yes. DirectX 12 can result in power savings or performance gains of 50 percent or more, according to Intel and Microsoft.
"This is like getting free hardware," Bryan Langley, a principal program manager for graphics with Microsoft, told PCWorld in March. "So if you're a gamer, and you upgrade to Windows 10, and you have that Iris Pro, it's like getting that extra kick. It may make your game go from not quite playable, to playable, from mediocre to awesome, from awesome to just out of this world."
AMD's Raja Kojuri also said DirectX 12 is like "getting four generations of hardware ahead" during the technology's unveiling in 2014. With the similar-to-DX12 Mantle enabled, Battlefield 4 saw performance leap by as much as 58 percent versus DirectX 11 results, though the exact amount varied greatly depending on the hardware setup.
You can kind-of-sort-of test DirectX 12's chops for yourself right now using 3DMark's "API Overhead feature test." Gordon Ung's walkthrough explains how to do that, but if you just want to get to the meat of the matter, DirectX 12's potential performance leap looks insane.
All that said, the most concrete DirectX 12 performance proof we have in hand right now are promises from industry vendors and results from a single, synthetic, theoretical benchmark focused solely on draw calls. There are a lot of reasons to be optimistic--heck, maybe even downright excited --about DX12's potential, but take it all with a pinch of salt until the first benchmarks from DirectX 12 games start appearing.
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