Well I've been planning on purchasing a GTX 750 ti, but I've heard people say that any pre 900 series Nvidea GPU's won't work that good with directx 12, they will be slower than directx 11. Is this true?
Async compute has been a feature of CUDA/nVidia GPUs since Fermi.https://www.pgroup.com/lit/articles/insider/v2n1a5.htm
NVIDIA GPUs are programmed as a sequence of kernels. Typically, each kernel completes execution before the next kernel begins, with an implicit barrier synchronization between kernels. Kepler has support for multiple, independent kernels to execute simultaneously, but many kernels are large enough to fill the entire machine. As mentioned, the multiprocessors execute in parallel, asynchronously.
That's the very definition of async compute.