I had great and fast sync when it was still Scrivener 2, on both old hardware and newer hardware, with iOS. The downloading File List step used to take about 20 seconds, and then syncing would be very quick. It was great, and I never had complaints. Then, after the upgrade to Scrivener 3, I suddenly had to wait about 5 minutes (15 minutes on older hardware!) to get through the Downloading File List step. The syncing itself of all the files, would be at the same speed as it ever was. Having to wait 5 to 15 minutes for every file I changed, even with just changing one line of text, really became annoying quick and it never went away. According to information on the forum it was a combination of the Scrivener 3 project format making a lot more internal files, and the API of Dropbox taking forever to detect changes when you have a lot of files. All good and well, but since 2017 or whenever Scrivener 3 came out, syncing has been annoying and time-consuming, although I never really had data loss (maybe once, but it was a small thing, that I could easily could get back because of backups). But the fact is: this Dropbox API thing, where the API takes abnormally long checking files, has not been fixed in 2 years, and it should let the team at L&L think hard and long at being so depending on a sync provider system that has been having it's own issues for a while now. (I'm sure they have, btw, I'm just on a little rant

I still have to check if with iOS 13 I can reliably copy Scrivener Projects manually from iCloud Drive through the Files App (I've had very quick and reliable syncs with iCloud Drive these days, and I have all my documents on there since I've replaced a my Macbook Air with an iPad Pro) back and forth and just completely leave Dropbox behind. I haven't done the checking yet, because of time and because I still hope one of these days it gets fixed with an update to iOS 13.2 or a Scrivener update.
Let’s hope that happens sooner rather than later.