P: Tif scans won't open after extensive Gen Ai removal work

The hardware is certainly powerful so the fault is probably not there. My M1-based MacBook Pro is much less powerful than yours, and it has no problem with scans of that file size. I don’t think 158MB is very large for a TIFF. I also edit off of external NVME SSDs, so there should be no problem there either. In other words, if I was having that problem I would be bewildered too.

I’m not sure what the hangup is with Camera Raw re-opening those TIFFs, but if I had that problem I might try opening and re-saving the TIFF file in other software such as Photoshop in case it writes it back out maybe differently enough to fix the problem. Yes, there is a risk that the correction metadata might be lost, so test that with a copy of the TIFF file. Sometimes I will also try re-saving a file through some of the free/open source photo apps.

Another option is to see if Lightroom Classic is any better at displaying that file, in case its code is different enough that it doesn’t run into the problem. 

I suspect the problem is somehow related to the edit metadata. Otherwise a TIFF file of that size should not pose any kind of a challenge on the hardware you’re using, where it should open quickly.

Is there a sample TIFF file that you would be willing to attach to this thread if you want to see if someone else can load it successfully? Because if no one else can, I would guess that someone at Adobe would want to look at that file in case there is a metadata-related performance bottleneck they didn’t anticipate.

Another perspective: The next time you try to load an affected TIFF file into Camera Raw, open macOS Activity Montor and watch the CPU and Memory tabs. Do either show a spike in resource usage when loading that file, like 450% CPU usage or does the Memory Pressure graph rise into the orange or red zone?

- - -

Now for an unsolicited opinion…

I sometimes do similar work (restoring scans of old damaged film). Although it’s very nice that Camera Raw/Lightroom Classic edits are nondestructive, a high number of retouching edits or masks in a single image can slow down the software because it’s always re-checking all of the edits in case it has to re-render any. I came to the conclusion that it was much more efficient to use the Remove/Heal features in Photoshop because once applied, they’re rendered to pixels. I do add those retouch edits to a separate layer in Photoshop so that the original is preserved. I’m under the impression that the Remove tool in Photoshop should be as good (maybe even better) than the one in Camera Raw, if its generative AI option is enabled.

This way, when I want to do a bulk tone/color correction session in Lightroom Classic (works the same as Camera Raw), the TIFFs don’t come with all of the performance overhead of many spot retouches, since those were rendered down to pixels in Photoshop. As a result Lightroom Classic/Camera Raw only have to track and re-render the tone/color/detail edits.

How big are the files in terms of size on disk?

For whatever reason external drives with macOS can slow to a crawl at times, so that's why i suggest copying one of the files to your internal ssd and then see if that copied file opens any faster in ACR.

To rule out the external drive as the cause, try copying the file to your main drive and see if that makes any difference.