You're at your file sync quota (and should be seeing a message to that effect). If you don't want to purchase a storage subscription (or use a third-party WebDAV server), you should disable file syncing in the Sync pane of the preferences. Currently, in large libraries, being at quota will cause some slowdowns as Zotero repeatedly checks to see what files need to be uploaded.

Then we'll need a Debug ID for a operation that's slow. It's highly unlikely that you actually need to force quit it — whatever it's doing, if you let it go it will almost certainly finish on its own — but if you're not on Windows you can also generate real-time debug output, which you can email to support@zotero.org with a link to this thread. That would work even if you had to force-quit it (though I'd guess that you would see it actually doing stuff).

1) Is D:, where you have your Zotero database, a local drive or a network drive?

2) Did you by any chance change the max pages/characters settings in the Search pane of the Zotero preferences?

In any case, I suspect you'd find that the 4.0 Beta would fix this for you, so I would try installing that. The slowdowns you're seeing look to be from background indexing of synced full-text content, and the beta is much faster at that.

Still lagging... Here is a new debug id: D1679058237

What kind of lagging are you seeing, exactly?

Since full-text indexing is much faster in the beta, it will try to index some large documents that it skipped previously. It starts doing that if you don't touch the computer for about five seconds while Zotero is open and stops if you touch the mouse or keyboard. I suspect if you just let it go for a few minutes without touching the mouse or keyboard it will finish and be fine after that.

This is almost certainly only going to be an issue as Zotero processes a backlog of your many large documents, which it will do if you just let it go for a short while. New individual documents should process fairly instantaneously in the beta.