Lightroom 7.3.1 & 7.4 get increasingly SLOW...

I wasn't going to post this 'cause it usually gets blamed on any OTHER than Lightroom, but it's bad enough I figure I may as well throw it out there...

Windows 10 Pro 5820, nVidia 970 GTX, 32 GB DDR4 memory, O/S, cache, temp, catalog on a 1TB SSD, images on pair of internal 5TB,7200 rpm drives in RAID 0.

Graphics card has been on AND off.  Makes no difference.

Automatically write XMP is NEVER on.

Catalog is around 70,000 images, regularly optimized.

Sync is always OFF.

Preview size is set to the size of the monitor (auto).

Have imported with Embedded & Sidecar AND Standard previews.  Makes no difference.

Have imported as normal .NEF files AND have converted to dng at import.  Makes no difference.

Have TRIED a SEPARATE catalog with only a few thousand images ON AN SSD.  Makes no difference.

Have used smaller Nikon D810 images AND larger Nikon D850 images.  Makes no difference.

As near as I can tell there are NO interesting errors in the Windows logs - I get the periodic ESENT information entry (like everybody else), and the BTHUSB warning periodically (again, like a significant percentage of Windows users).  NOTHING in either application or system that indicates any problem.

Start Lightroom (btw: at startup, before touching a single key or image, LR typically uses 5+ GB of memory - I presume this is normal with the current version).  Select a dozen images in Library and hit the space bar to display the images filling the center screen.  Rapidly hit the right arrow to walk through the images and make sure they're loaded.  Then take a quick look at each image to make sure the subject hasn't moved.

If set is good, send them to Helicon Focus if it's a focus stack, or do a Lightroom Photo Merge if it's a panorama or HDR.

Rename the output image so I know which set it is.

Select the next set and repeat.  Sets may be anywhere from 3 images to 20+.  Near as I can tell, makes no difference.

Occasionally, take an image into Develop and make a few simple, gross adjustments just to see if it's worth keeping.

So, do basic culling and checking plain, ordinary images - the same kind everybody out here (I suspect) has thousands of.  The kind of light operation that should be extremely fast - no heavy editing, no large number of steps in history, no using brushes or anything else that's always been painfully slow.

I suspect it has nothing to do with focus stacks or anything else - if I go through a day's shots from an Air Show - maybe 2000-2500, the same thing happens as I move back and forth, viewing in grid or in loupe, occasionally switching to 1:1.

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It DOESN'T appear to be linear, but checking the process display in Task Manager periodically I can see the memory used by Lightroom increase.  Usually within a half hour it'll be at 11GB or more, and I've seen it in excess of 16GB.  I'm NOT sure which operations cause the big jumps in memory used.  But as near as I can tell, it never gives up any of the memory 'til I shut it down, or crash if I'm lucky when it won't exit, or crash the whole system a few times when it wasn't possible to do ANYTHING else.

As I repeat the culling operation, Lightroom slows down.  With 10 - 30 minutes, even an action as simple has pressing the arrow key to move to the next thumbnail in the grid display gets VERY slow - more than a second.  Setting a color label is even SLOWER, and if you select the thumbnail, hit the key for the label and move BEFORE it eventually changes the color label (which may be anywhere from 1 to several seconds), it'll change whatever image the cursor is on rather than the one you were trying to change - it's that slow.  I have had images for which I selected the filename in the metadata tab in the library and had LR sit there and spin for a VERY long time - well in excess of 10 seconds BEFORE it actually selects the filename.  Same thing for selecting "File" from the file menu to try and shut LR down cleanly - once it gets really slow it'll sit there with the blue circle spinning for a very long time before the menu opens.

EVENTUALLY, figure somewhere around 30 minutes for 7.4, and maybe 45 - 60 minutes for 7.3.1, Lightroom loses it's mind - there's another topic I entered about the "Lightroom chaos" with 7.4.  It's bad enough that I reverted to 7.3.1, which SEEMS to take a little longer to go crazy, but gets just as slow, just as quickly.  I'd put a link to the other topic if I had any idea how.

Generally, to avoid the chaos condition, I shut down Lightroom when it ALMOST gets to the not responding at ALL condition.  If I Select Develop with an image, and it STILL hasn't switched to the Develop screen in 5 - 6 SECONDS, it's time to shut down.  Or if I select a half dozen images and try to have Lightroom do a panorama or HDR.

I KNOW it didn't exhibit this behavior with ANY of these images PRIOR to 7.0, but I'm not sure WHEN in the various V7 versions this got bad enough to be a major impediment to editing.  Most of the early versions of V7 were just slow ALL the time so I don't know if they also got increasingly slow, but I believe from 7.2 forward it's been significant and very perceptible.

AND, FWIW, EXACTLY the same thing happens with the laptop, but it's slower to begin with (i7-6700, 16GB of memory GTX 970, O/S, catalog, cache on an SSD) and the images are on an external, 7200 rpm, 6TB HDD so I figured I'd see if the same thing happens on the much faster desktop.  It does.

Anybody have any useful ideas for things to try?