P: TIFF compression settings have no effect when Lightroom sends image to Photoshop for editing

OK, that's finding them, though you could always sort using my Search and Replace plugin, and doesn't do the updating.

Take a look at these rough screenshots at https://www.dropbox.com/s/xebqmvhvyafgvkd/droplet-setup.jpg?dl=0 . In PS, you create an action like the one in the top screenshot. Then use PS's File > Automate  > Create Droplet with the settings shown - the droplet/action can be named/stored as you want, but be careful on the right hand side.

With the droplet created, you can either drag photos onto it using Finder and wait while PS runs the droplet, or set the droplet as an external editor in LR and choose pics from there, editing the original when you're asked to choose.

Test on a couple of items first.

@john beardsworth

I did what you suggested, but finding the files to resave took some command-line foo as LR doesn't have a query option by file size.

find . -type f -size +100M -name '*tif' -mtime +15 -print0 
  | xargs -0 stat -f "%z %Sm %N" -t "%Y%m%d" 
  | sort -k1rn -k2rn

(Line breaks are shown for legibility only)

The above command finds all files with the extension tif that are larger than 100 MB and not modified in the last 15 days. It then sorts them by size and then by date. Using this command, I was able to find all the images on the disk, and then I simply found them in LR by searching by the names.

Use Image Processor Pro + Bridge.

How did you automate the re-save, that’s exactly what I’m looking for.

Do you really mean a "script"? If so, in PS, create an "action" to run that script and save the action as a "droplet" from the File > Automate menu. This file can be treated by LR as an external editor, so you select affected TIFs in LR and edit with the droplet.

For large files that use multiple Smart Objects and occupy around 2 GB on disk, I'm seeing a space saving of 500MB or more. But it is simply not practical for me to open up every image I ever saved as TIFF in PS and save it again by compressing to ZIP, so I'm wondering if it can be scripted. FWIW, I'm running latest macOS and PS. I know how to find TIFF files that are larger than a threshold value (say 500 MB) using a script, but I don't know how to "resave" them using the same script.

This not being fixed after 8 years is beyond ridiculous.

See the following link: https://petapixel.com/2020/04/13/this-lightroom-bug-could-be-costing-you-tons-of-storage-space/
 April 13, 2020 I just tried this.  Set the Lr Classic CC external editing preferences for Adobe Ps CC to TIFF with compression None.  and then set compression to ZIP.  In both cases the resultant TIF was the same size.  Both my Lr and Ps are up to date.  Why no compression?

Lightroom CC 2015 & Photoshop CC 2014

I've set in LR: Preferences - External Editing -> Photoshop CC -> Compression TIFF 16bit to ZIP.
Inside LR when choosing to edit a file in PS and saving that file after I've edited, no compression is applied to the TIFF file. The result is always an uncompressed TIFF file.
Which makes the LR preference setting completely useless.

It's unbelievable that this remains broken.  I have a note taped to my monitor to re-save folders of TIFF images nightly using the Automate->Batch function of Photoshop, but boy is that a pain in the ass.