Yup, both of them take at least 1-2 seconds to respond to mouse movements. Could you do WebGL and blend into ray-traced? Have you seen what Octane/Brigade (not exactly same tech, but close) can do in realtime? https://www.youtube.com/watch?v=BpT6MkCeP7Y (you probably have)
Anyways, Brigade is coming to cloud as well on AWS GPU instances, so it would probably be easy for you to just use their solution integrated within your modeling tool then.
It is possible to integrate Octane/Brigade into our offering. The renderer, be it ThreeJS or V-Ray or Octane, is already a separable model from our tool.
Here is the same scene using our ThreeJS renderer (e.i. WebGL) integration:
In fact, we & Klaas Nienhuis have done something like that with this interactive demo. The lighting is done via V-Ray but it is baked into the WebGL textures:
Anyways, Brigade is coming to cloud as well on AWS GPU instances, so it would probably be easy for you to just use their solution integrated within your modeling tool then.