I mean if you already know what all of your materials look like beforehand, then you probably aren't the target audience for this functionality. I do hear what you're saying about the IR being slower - maybe since I'm coming from snappier GPU based renderers I'm used to having the render view active most of the time while working. I would be happy with it either working within the viewport or the IR, I'd just like to be able to hit a key to preview how a texture is mapped, like most other renderers.
I'm curious what peoples' workflow is like in Corona without this kind of feature? If you are customizing a material or creating one from scratch, how do you preview what a map like bump or roughness will look on an object (aside from actually waiting for the render fully resolve in the IR, then adjusting, then letting the render resolve again, etc)?