The synchronizer is pretty slow when uploading multiple files at the same time. It takes around 1 minute to upload a 10MB file then start the next one, but my network usage reaches the peak for only 2~3 seconds, then drops to nearly zero for the rest of one minute. I think the network bandwidth is not the bottleneck.
Is it possible to make the huge loop parallel? let's say uploading 5 or 10 files at the same time.
If someone can make a pull request, why not, but I'm not sure it can be done with other sync targets, because there might be Joplin-specific behaviour happening on Joplin Server side (I forgot exactly how it's done)