Hello,
with DropBox, the number of HTTP requests within a certain time is limited. So if you upload many small files, it can be very slow. Uploading to Amazon S3 can probably be 100 times faster.
None of the modern cloud drives allow really many uploads in a small period of time. They vary slightly, but they all have the same problem of "rate limiting", and sometimes slow processing of HTTP requests. You can increase the number of files being copied in parallel, on the Files tab sheet in Syncovery, but you may risk getting penalty delays.
For speedy uploads, I think you need dumber cloud storage, which includes Amazon S3, Microsoft Azure BLOB storage, Rackspace Cloud Files, and Backblaze B2.
I think Backblaze B2 will be the most cost efficient, and it has the advantage of being able to retain the files' Last Modification timestamps. The other "dumb" storages do not have that capability.
Of course there are many S3 compatible storages, which may help save some money. The most famous one is Wasabi I believe.
Among the more modern cloud storages, I think PCloud is the fastest one for uploads.