Actually I purchased Max 2021 not long time ago and worked only with a few projects in it. Today I spent some time inspecting the scenes I 'm working with at the moment and finally found the problem point. It is a group of objects and some problem is hiding probably in their material. I've uploaded the max file which contains those objects - multiple piles of books. File name is 1611966917_books-bug.zip
All those books have one multi/sub-object material with 57 very simple sub-materials assigned to them.
The steps to reproduce the issue:
1. Create Corona material, change something in it - preview updates instantly.
2. Assign this material to the box or to any other new or old object, change something in this material - Max is hanging for a second, after that material preview updates.
3. Create 10 copies/instances of those piles of books. Change something in that material - now Max is hanging for 10 seconds, after that material preview updates.
4. Create a new material, assign it to all those books. Change the material you created before - problem has gone, material preview updates instantly again.
As I mentioned before it doesn't matter are those books hidden or shown in viewport, are they renderable or not. Viewport settings do not affect this issue.
Before I purchased Max 2021 (finally) I worked on my old Max 2014 and I'm able to make the same test with exactly the same objects - there is no this problem with Max 2014 and never was before.
I suppose this issue related to the problem I described in "Corona Renderer 7 for 3ds Max - Daily Builds Discussion" topic
https://forum.corona-renderer.com/index.php?topic=30877.90But with Corona 7 DBs it affected stronger. Unfortunately I don't have time to check it now, but I can do it later if you want.
Windows 10 pro, 2 x Intel Xeon E5-2660@2.20Hz, 62GB of RAM, NVIDIA GeForce GTX 1060 6GB