Thanks for your followup. I just checked and it isn't running in Rosetta mode.
A related question: When you use Selection Based Processing (SBP) does LPX create a new audio file or does it alter the original? I ask because I noticed that as soon as I applied SBP to a region the LPX project file grew to a much bigger file size than usual.
I might have had the original audio file open externally in iZotope RX at the same time—which probably forced LPX to create a separate audio file thus increasing the project file size? I only tried SBP on that one region exactly because I thought something was wrong, hence this post.
I encountered an unusual problem with this project which I think might be related to doing SBP while having the audio file open externally in RX. Every time I did some fix in RX and returned to LPX the progress bar under the display kept loading and loading over and over for at least 3 minutes. Normally, I only see this progress bar once per audio file but this time it seemed to show a progress bar for each region! I've never seen that happen.
Eventually I couldn't even open the project file. I was almost panicking because I had already worked on it for about 12 hours. Luckily I came across a video that helped me hack it so I could "reconstruct" the project: