Juraj - So I just did a quick test and on a super heavy scene with tons of forest pack my 3990x was dropping to 9-13fps during IR, and the 3970x was staying more around 18-23fps. Not an exact test but relatively true to my experiences. Even outside of 3ds Max, I find the 3970x to server me a bit better. The 3990x would randomly 'bog' and I'd get super frustrated with it. Pretty subjective, I know.
The backstory is I had been using my 3970x as my workstation since I built it and using the 3990x as my render node and that was working well. But then a couple months ago I moved into a different office space and decided to switch to my 3990x into the role of workstation because I thought IR would be faster. I then noticed things being pretty sluggish and my productivity dropping and I thought it was a bug with Corona or my windows install or something. I did all the usual things: Control flow gaurd, real time threat, defender, nvidia drivers, etc. all done and investigated to the point of nausea. I even started a thread on it over at 'I need help.' For a long time it didn't occur to me it might be the different system, which in retrospect seems pretty dense on my part :-) Eventually I connected the dots and switched back to my 3970x and bam, happy again.
Keep in mind that's totally anecdotal and it may well be something about my build. For example, I just built another 3990x as a render node and it runs slightly faster than my other 3990x by a few hundred mhz when used as a render node, but I think that's because the new build still has the case door off (literally just built it) and is running cooler. I have NOT tested that new 3990x as a workstation, maybe it's just as fast as my 3970x. ALSO, the original 3990x has 128gb of RAM, whereas my 3970x only has 64gb of ram (all running quad channel of course).
My guess is the difference I found between the 3990x and the 3970x is due to one of these:
1 - Idiosyncrasy of one of my builds causing this difference
2 - Higher core speed on 3970x helps just enough to be noticeable
3 - Something in the backend of the chips that relates to IO
4 - Windows and super high core count causing issues
5 - 64gb of ram on a third gen threadripper just runs faster than 128gb
6 - It's all in my head
I would argue #6 is the least likely, but just throwing it in there :-) Anyway, hope those experiences are helpful! Don't build a 3970x just because of me, but maybe you could test another chip for free somehow.