Once I open LR it takes a iong time for certain functions to work,i.e., "Edit in..." Can this be corrected? LR (13.5.1); Mac OS (12.7.6). Cannot upgrade Mac OS.

In my opinion, you should not have to move photos from one disk to another to get LrC to work properly. However there are exceptions, for example if your disk where the photos are stored is very close to being full. You say 10TB are used out of 12TB, that doesn’t seem to me like it is close to full, but I do not use RAID disks so I don’t really know. 

In my opinion, a more likely explanation is that your RAID disk LAN is not configured properly. What kind of LAN? Who makes it? How do you know the LAN is working properly and how do you know the RAID disk is working properly? Do you get tools to test speed?

Another thought is if the files on your 12TB if you have a humongous number (let’s say 10,000) of files in a single folder (and no subfolders), or even a humongous number of files in one folder not counting subfolders, that could be a problem. Does this sound like how the photos are stored on your RAID drive?

What happened if you moved the image file to the internal disk. LrC does have sometimes issues with handling images that stored on NAS or external drives.

How is the NAS connected to your computer in terms of performance?