How to best optimize high-res native .R3D files to be warped with STMAPs to make lower res proxies?

Basically I am trying to setup a workflow that can input and read 3 streams of native dragon footage, warp and stitch them with STMAPs and output them as a lower res proxy file. (from 10k down to 2k)

What kind of processes do the stmap and reading r3d nodes get accelerated by?

From what I understand once you go to lower debayering Rocket-x cards have no benefit over dedicated gpus. What accelerates stmap rendering? Is it a CPU hog or is it also GPU accelerated?

Thanks