Don't worry, it's never fast for me either :- ) I only use it when rotating single object isolated and even that I have to do in almost empty scene otherwise the interactive will just hang-up for ton of time before starting (regular rendering starts instantly).
Didn't find sub-sampling to help much since when IR is slow, this will further obfuscate the image in buffer by making it unrecognizable during those 2 seconds freeze. Without sub-sampling, I can at least directly compare changes.
Sometime all those hang-ups in bigger scenes make regular hit/quit rendering a lot faster to facilitate iteration for me.
You can check 'force path-tracing' in settings so that IR will always be PT/PT which will avoid the UHD cache re-comp, but to me even that doesn't help. I think 3dsMax itself is at fault of moving all those datas. Sometimes I feel like maybe using all those 8k textures, subdivided surfaces,etc.. isn't best idea because it forbids me from using interactive, but then again, I use Corona purely to be able to render big, complex, high-res scenes. Otherwise I would just use Unreal.