I did some further testing at home between various cloud providers and between NetDrive and ExpanDrive using 22 JPEG files totalling 107MB.
NetDrive performs a lot better than ExpanDrive, particularly with multi-threaded transfers (e.g. robocopy /mt) where it's over double the transfer speed with Google Drive, at least going over my ISP.
The OneDrive API seems to be suffering API issues today as both products kept giving I/O errors when trying to copy from OneDrive. The Windows built-in method still worked.
Dropbox was the slowest. Even with multi-threaded transfers, the total throughput was about 0.55MB/s with NetDrive and half this speed again using ExpanDrive. With linear file transfers, it only went about 0.37MB/s in NetDrive, not much better than OneDrive the last day it worked.
Amazon Cloud Drive did not fare much better with 0.62MB/s for the same test. With linear file transfers, it only went about 0.46MB/s.
Google Drive was the quickest in NetDrive. With the same file set, the throughput was about 1.81MB/s. I'm sure my ISP is somewhat limiting this.
OneDrive for Business was a bit of a surprise. Despite having by far the worst sync product I've come across, both its API and Windows bulit-in method perform quite well with file transfers. With NetDrive, I got 1.45MB/s with multi-threaded transfers of the same file set. With Windows Explorer built-in network drive mapping, I got about 1.73MB/s.
Surprisingly, NetDrive did not seem to perform much better with multi-threaded FTP transfers and certainly nowhere near the performance of a dedicated FTP client on my home ISP. With a linear transfer, I got 1.2MB/s, only increasing to 1.39MB/s with a multi-threaded transfer.
With FileZilla, I got 2.33MB/s in a linear transfer and 2.55MB/s with a multi-threaded transfer, which I'm sure was the limit of my ISP at the time as 20Mbps would be what I typically get in a speed test during the day.
My workplace is expected to get VDSL in two weeks time, so I'll wait until then to do more in-depth testing between the cloud providers. Although my ExpanDrive trial will end then, it's pretty clear at this point which one performed better.