I don't know if anybody is still following this, but...
First, if anybody wants to see about the system here, check back to this post:
Earlier System Description
Since that time, the Studio macOS version has been updated several times and now is running on the latest version of Sequoia. (Yes, I am 🐔 about changing to Tahoe - I couldn't find even one feature that I imagine using in Tahoe that isn't already available in Sequoia or from an available third party app.)
Anyway, a couple months ago I got really tired of the spinning drives in the RAID box, umm, spinning all the time. Very annoying, plus probably not great for the drives themselves. After some research, including asking the SoftRAID people, it appeared that the issue was Spotlight indexing the drives. Over and over. And over. Not an unusual problem, apparently.
So, I tried the various solutions such as deleting Spotlight databases, reindexing, and so on. They all helped - maybe there was half the audible indexing. But, still annoying.
Fast forward to two weeks ago. I couldn't take it no mo. So, I prevented Spotlight from indexing the three volumes in the RAID drive. That seemed reasonable. I almost never use Spotlight to find files on these volumes. There's no system files on these volumes, so that isn't an issue. I left Spotlight indexing active for the internal Studio drive and the external Time Machine backup drive, which is an APFS formatted SSD.
That certainly stopped the incessant churning. I found a good app that will search for any files I need on these RAID volumes, if needed:
EasyFind
OK, what does that have to do with the subject of this thread?
I noticed a couple days ago that when I went to look at a large folder, the results came up very quickly. As in, a couple seconds. Much like the good ole Mojave days. I've tested this since, and it seems to be holding. (That could change this afternoon - who knows?) More than 5000 files in about 4 seconds - instant after it's been opened once. (An Onyx system maintenance resets that, of course.)
I'm not saying that this solution would work for everybody, but so far it's been really good for me. In fact, I even removed Spotlight from the Menu Bar. I haven't missed it. My goal wasn't to speed up reading large folders in the RAID system, but I won't complain that it seems to work. At least for the past week or so.
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An alternative, that I've been too lazy to try implementing, might be a launchd script that turns Spotlight indexing on for these volumes at some arbitrary time, like 3AM, and when complete turns the indexing function off again until the next morning. That's for somebody more skilled and ambitious than I am.