In other words: Don't necessarily expect miracles. Know what your hardware is, know where your system's bottlenecks are, and don't be surprised if Mantle fails to be a magical cure-all for your PC's gaming performance.
Someone still loves you, DirectX
Mantle still faces an uphill battle for adoption, even if it does improve performance.
Developers aren't going to suddenly stop making most every game in DirectX — at least, not yet. There are too many PC setups out there, and Nvidia and Intel are (much to AMD's dismay) still the dominant graphics card companies as far as PC gaming is concerned.
While AMD says Mantle is an open platform — in other words, "Sure, Nvidia could take advantage of our new API" — don't expect to see Nvidia crawling to Mantle's door anytime soon. And that's not just because of longtime rivalry between the two companies; the architectures behind AMD and Nvidia's cards are so different that it's doubtful Nvidia could even use the system without huge rewrites.
As a result, developers will have to opt in to Mantle, including it alongside something more popular like OpenGL or DirectX. Otherwise they'd lose a huge section of the market. Mantle will presumably always be an "also-ran" API instead of a primary focus, unless AMD's successful push into the console market is a much larger factor than anticipated.
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