honestly i dont think this is CPU related at all but simply the IR.
People have been reporting slow downs and extremely frustrating loading times with IR. Myself I've been dealing with the IR and what I've seen is:
*IR works good first time you press it
*breaks and loads rediculously long when pressing it a second time or changing material-properties for example. The frame buffer will be stuck for minutes. Once I just waited it out and after 15 minutes it decided to show and updated the buffer, like it had rendered 50 passes in 1 go.
*quickly press "normal" render mode - cancel when loaded
*press IR again and should go smooth (will break again second time)
So i'm juggling in between IR and normal-render mode all the time
Above that you wanna save alot with IR cause it might freeze your max + I have the issue of when pressing IR and it's doing its thing, when going in a different windows-program, the max window starts bugging out, like the buffer and other windows in max come on-and off, they keep going to the foreground, and are being pushed to the background - back and forth, making you unable to press save, cancel or anything else for that matter. In other words: you can't do anything but force-close the .exe and start over.
I dont know if this is max or corona related atm.