Denoise takes 30 minutes to run, and Lightroom is reading every image in my catalog.

Anyway, I fixed the issue by using a backup catalog from 6 months ago and importing the missing photos from the problematic catalog.

After importing the new photos, Lightroom was still doing the same read behavior but it was only doing the last 20,000 photos from the import and not the 500,000 photos from my entire library. After letting it runs all night, it stopped with the constant reads. No more idle network traffic, no issues with the Lightroom process not terminating on exit, lrcat-wal deletes on exit, Denoise takes the usual amount of time. So clearly something was wrong with the catalog that didn't show up in an integrity test. 

Please post your System Information as Lightroom Classic (LrC) reports it. In LrC click on Help, then System Info, then Copy. Paste that information into a reply. Please present all information from first line down to and including Plug-in Info. Info after Plug-in info can be cut out as that is just so much dead space to us non-Techs and it takes up vast amounts of scroll space making the reply less readable and less likely that others will bother with your post.

 files on a NAS connected with 10GbE

Where the files you are referring to are the photos (not the catalog)

Maybe I am reading into this wrong. I am trying to connect that you use a NAS with that screenshot. When I google IronMountain, I do not see a NAS but a offsite data storage. So is your NAS by IronMountain? Yes I know SMB would be common to access accros that NAS.

Not a big fan of using NAS for active work in Lrc (including just photos), just for archiving.  So a huge hole in my lessons learned.

Weird new finding: If I kill Discord and Spotify, Lightroom closes. If I have either of them open, Lightroom.exe will never close. 

Another issue: When I close LrC, the UI closes immediately but Lightroom.exe is still running and keeps hammering the network for a good 5 minutes. Then it slows down and stops and the process just sits there idling until I eventually kill it. 

One more thing, I noticed that the .lrcat-wal file is growing constantly while it is accessing all of the catalog files. It's currently around 1GB in size. They aren't removed when closing Lightroom.

Also, I reinstalled LrC and also created a new catalog and the issue persists.