It is hard to say how fast it syncs since it doesn't have a specialized app for storage on Mac and in iOS it is just Files (kind of Finder app). Photos do sync reasonably fast, give it few seconds, so do Keynote and Pages files.Those of you who use iCloud Drive instead of Dropbox — does it actually sync files in a reasonable amount of time for you? I find that even a 10k file on my desktop can get stuck syncing for hours. This isn’t an isolated phenomenon. It’s very common.
I just tried to sync a PDF file uploaded from desktop and saw it appear in Files in a second or so.
So yes, experiment is successful. As for Dropbox, I liked it first, but its meager 2GB of storage is very limited, and paid is very expensive, so I have iCloud and very happy, also with Office 365 subscription, I get a huge OneDrive as well. I don't use Google or other storage apps, just iCloud and OneDrive.