Correct, and this will be true of any initial sync - though a remote sync location will obviously take more time than a local one.
Actually, Bonjour also syncs automatically. There isn’t a difference between Bonjour and non-Bonjour syncs regarding this. 
I read through the whole thread and it was mentioned that I can use Bonjour parallel to cloud syncing.
I definitely want cloud syncing, because MacBook, iPads and iPhone are not always on the same network with the always running “master” iMac when I need access to the DT documents. - But still, speeding up the process in the casethey are on the same network is attractive.
So when on the same network and everything’s switched on simultaneously, does Bonjour work as automatic as cloud syncing? Or do I have to remember to press sync? And can I use it indeed parallel to cloud syncing?
I never had any problems with using box.com for sync – as long as two devices synced, items added to one would show up on the other without delay.
Now that Box are dropping WebDAV, I switched to using a WebDAV account on my home Synology box. This gives me fast sync when both systems are on my home WiFi, and still works across my wireless Internet if I forget to sync before leaving the house. (LTE data via T-Mobile, IPv6.)
Does iCloud sync allow me to store files on iCloud with also having them be downloaded to the iCloud Drive folder on my computer?
This is like the Dropbox “selective sync” feature – where the files are on the cloud service but don’t take up space on my local computer.
I am interested in cloud sync, but I don’t want my DT databases to take up twice the space on my computer, once for the database and once for the cloud drive sync store.
DTSync is done locally, then iCloud uploads to Apple’s servers, then to devices using your Apple ID. It should clean up after itself, but that’s under iCloud and macOS’s jurisdiction to handle.
I’m afraid I don’t understand that answer.
If I have a 10 GB database, it uses 10 GB of cloud storage. My question is, does using iCloud sync place that copy in my iCloud Drive folder, so my 10 GB database takes up 20 GB of space on my hard drive?
Your database cannot be in a cloud-synced folder, so the database can’t use 10GB of cloud storage.
Sync does not copy your database into a sync location. It syncs raw DEVONthink-specific data for use only with DEVONthink and DEVONthink To Go.
But will your data consumption on iCloud for sync be 10GB? Roughly speaking, yes.
Okay, it copies DT-specific sync data. The format is not my point here. That said, the cloud-based sync store does use essentially the same amount of data as the original databases. So a 10 GB database produces a roughly 10 GB sync store.
Say I use Dropbox as a sync store. DEVONthink creates a folder within Dropbox and sends over a bunch of data to it. By default, Dropbox syncs all folders to my desktop. So by using Dropbox sync, a DEVONthink database uses twice the space on my hard drive – it has the database file stored locally, plus whatever data it has placed in the Dropbox sync store which gets added to my Dropbox folder.
However, Dropbox has a “selective sync” feature which allows me to disable syncing of specific Dropbox folders to my computer. So I disable the DT sync store folder in Dropbox selective sync. Now the DT database only uses up its original size on my hard drive. The sync store is on Dropbox, but does not get synchronized to the Dropbox folder on my computer because of selective sync.
Make sense so far?
Now I want to know if iCloud works the same way. I haven’t used iCloud much at all, and not with DEVONthink sync. Ideally the DT sync store would be stored on the iCloud Drive but not downloaded to the iCloud Drive folder on my computer.
Actually, that’s incorrect.
DEVONthink does not create a folder within your local Dropbox folder and sync to it. You are supposed to disable the /Apps/DEVONthink Packet Sync folder from the Dropbox application’s preferences, or Dropbox will import this to your machine.
Our sync engine does NOT use the Dropbox application at all. In fact, you don’t even need it installed on a machine to use our engine with Dropbox.
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As I said previously:
DTSync will sync to your local machine (which is why it clears quickly in DEVONthink). iCloud then uploads to Apple servers on its own schedule (which we have no control over). When iCloud is done doing its own syncing, it should clean up the local sync data, hence not consuming space locally for the sync data it uploaded.
This is exactly what I’m saying. That’s the selective sync. This is the whole point of what I’m saying about Dropbox, and asking about iCloud.
This is what I didn’t know, and what I was looking to learn. With Dropbox, I have to manually exclude the folder from syncing. With iCloud, it is evidently automatic.