Can you go to Tools → Developer → Run JavaScript, run the following code, and let us know the output?

return await Zotero.DB.valueQueryAsync('SELECT COUNT(*) FROM itemDataValues') + ' ' + await Zotero.DB.valueQueryAsync('SELECT COUNT(*) FROM itemData') + ' ' + [...await Zotero.DB.queryAsync('PRAGMA index_list("itemData")')].map(x => x.name)

You should temporarily disable auto-sync from the Sync pane of the Zotero settings, restart Zotero, and let it run for a while — overnight, even. Then try running that line.

1107787776
782208
0|itemData_valueID|0|c|0
1|itemData_fieldID|0|c|0
2|sqlite_autoindex_itemData_1|1|pk|0

Separately, I left this group months ago and don't want to keep syncing with it, but if I disable sync in my settings, will that disable syncing with my RSS feeds?
https://s3.amazonaws.com/zotero.org/images/forums/u13382599/qjsk3tgmm6rk5op4ppmf.png

Could you close Zotero, run this from Terminal, and let us know the output?

echo "SELECT COUNT(*) FROM items JOIN libraries USING (libraryID) WHERE type!='feed'; SELECT COUNT(*) FROM items JOIN libraries USING (libraryID) WHERE type='feed';" | sqlite3 ~/Zotero/zotero.sqlite

And then you can try this:

time echo "DELETE FROM itemDataValues WHERE valueID NOT IN (SELECT valueID FROM itemData);" | sqlite3 ~/Zotero/zotero.sqlite

This is the command that's slow within Zotero, so this might take a while to run. When it finishes, let us know the output (which will say how long it ran for).

Then open Zotero and see if the problem is fixed.

Finally, close Zotero again and run this:

echo "VACUUM;" | sqlite3 ~/Zotero/zotero.sqlite

After that, let us know the new size of zotero.sqlite.