Mantle heavily reduces the CPU’s workload
R9-290X + FX 8350 Base Build:
As the speaker discloses after the demo (at 37:18), their CPU is actually hardly even being used. The developer presented the audience with a before-and-after frame analysis of how Mantle affects the system’s workload in comparison to DirectX. In the frame analysis taken from the game running with DirectX, we see a chunk of the CPU being used by the driver thread. In the second (the one where Mantle was used), that thread is simply gone.
Not only did it free up a lot of CPU time when switched to Mantle, but Mantle itself took only a fraction of CPU time to function as opposed to DirectX’s required driver thread. This means that developers have the option to go one of two ways if they use Mantle: they can utilize that unused CPU time to improve their game, or they can simply drop the CPU requirement.