When copying a large number of files, I frequently notice that, with four files are moving at any one time, one of them is moving much slower than the rest. There's an example screen shot at /a/oTwA55y on imgur dot com.
What is your rclone version (output from rclone version)
You may however also be limited by your cloud storage, so do it in small steps and with some caution.
I use OneDrive and it will not do more than 4 transfers at the time, if I set --transfers higher then OneDrive will start throttling my account and actually make my transfers much slower. (Try searching the forum for OneDrive throttling).
I don't know Jottacloud, but parhaps it only allows 3 transfers at a time - and what you are seeing is light throttling.
Probably best to test your Jottacloud, LAN and SFTP individually while copying local data. That will make it easier to find the storage (or network component) limiting your transfers.
I'll get a log as soon as I can but if you look at my unhelpful screen shot you would see that the current transfer is quite large and has several days to run. I'm reluctant to stop it without a known-useful fix.
The source this time is a large Samba volume on my LAN. But I also see this behavior from a local SSD and from a locally-attached USB3 spinning-rust drive.
Duplicati used to support Jottacloud as destination, but perhaps not with the new authentication methods? Or do you deliberately use a local disk as destination and want to use rclone to upload a second copy to cloud?
In my experience I can set transfers quite high, even as high as 16, and see increase in total upload speed. Depending on the data, of course. Don't think it leads to throttling.
I'm throttled at Jotta because of total volume but this problem is not restricted to Jotta. I am not throttled at Lima. And certainly not on the QNap in my basement.
Also: in a throttling situation, I would still expect the four transfers to be running at similar-ish speeds. Not three at X and one at 0.1*X
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