I'm not sure you understand the nature of this feature. There are 2 main types of tasks for a GPU, graphics and compute. D3D12 exposes main 2 queue types, a universal queue (compute and graphics), and a compute queue. For Ashes, use of this feature involves taking compute jobs which are already part of the frame and marking them up in a way such that hardware is free to coexecute it with other work. Hopefully, this is a relatively straightfoward tasks. No additional compute tasks were created to exploit async compute. It is merely moving work that already exists so that it can run more optimally. That is, if async compute was not present, the work would be added to the universal queue rather than the compute queue. The work still has to be done, however.
The best way to think about it is that the scene that is rendered remains (virtually) unchanged. In D3D12 the work items are simply arranged and marked in a manner that allows parallel execution. Thus, not using it when you could is seems very close to intentionally sandbagging performance.