Hi guys,
speaking from my own recent experience, I personally had quite an issue with IR in Corona 4. It was so annoying I just downgraded to C3hf2 for that reason, maybe it was due to some things still being on legacy settings, but as I needed it to work afap I just decided to finish the project in C3 instead.. I spent almost whole week trying out what the culprit was, couldn't identify it (re-saving layer by layer etc.).. But it was really very very unresponsive, like waiting several minutes for IR to start reacting every time I moved the camera or changed any settings, and I still had a lot of RAM available..
Although I think it may have been due to quite heavy CoronaDistanceMap usage in mix with the CoronaScatter and proxies? Maybe that is something you could try to recreate to investigate this further, like a lot of various object/proxies in distance maps + scattering.. I was definitely less optimizing things in the past on my 32gbs workstation and IR was working much smoother.. with much higher polycount at that.. One more thing that comes to mind was that maybe I must update the older proxies to "newer version" as well? Could that increase the performance?
I can try to give an update on the situation after I start a new scene from the scratch in C4, but that can take some time .. :) But well anyway, C3Hf2 have basically no issues with IR while C4 working with same scene is hardly ever responding..
Anyway I would be very glad if you make this a priority as fast IR is a must in nowadays standards..
Cheers