A few to look at are Octane, iRay and Arion which all make use of Cuda cores on Nvidia cards. I haven’t used the Razer Core but if speed is your main goal and you’ll be on a laptop, I’d recommend an external enclosure for multiple GPUs or build a dedicated tower PC with multiple PCIx slots, enough power and space+fans to keep it cool.
Rhino v6 is slated to also by default have GPU accelerated raytracing in the viewport as the “Raytraced” display mode. This is using the Cycles engine behind the scenes but integrated for the new materials and render settings in v6. Give it a try too when you can and let us know what you think.