I’ve spent a year periodically searching for a solution to my serious lag in Max when renering a progressive IPR. This solution seems to have solved it for me. I use a program with which I can assign which processes a particular program uses. I let Max use all but 4 of my 32 processes. When setting it up for Max I just excluded Max from the first two processes and the last two processes for no particualr reason so that other programs could utilize them. I didn’t realize that there was a priority to the processes that was that important. I reassigned Max back to the first two and and excluded it from others in the middle. Like magic, I now have a pleasant experience using Max while IPR is running. I’m thrilled to have found this. Thank you @3d-laboratory .
So if others are having the issue and even if you don’t use a program to manage processes, try making sure that Max has exclusive use of the first process and see if it solves the issue for you.