Can you try doing an upload not via the mount so, rclone copy file.bin 1fichier: and see if it uses the same amount of CPU.
I suspect it is the whirlpool hash 1fichier uses - it is extremely CPU intensive
Here is MD5 for comparison
$ time rclone hashsum MD5 /tmp/1G
And here is whirlpool
$ time rclone hashsum Whirlpool /tmp/1G
So my laptop can do 1G / 31.098s = 32 MByte/s for whirlpool per core.
Can you try this test on your machine with a large file?
Thank you for the fast answer. I will test your suggestion when my current transfer is completed.
So first I tested the hashing algorithms ~6.4G file:
~# time rclone hashsum MD5 OHO-052.mkv
~# time rclone hashsum Whirlpool OHO-052.mkv
As you suspected the whirlpool hashing algorithm is very slow in comparsion to MD5.
I also tested rclone copy and that uses (all processes together) ~200% CPU at ~200 Mbit/s with the same file.
When I try an upload via the website of 1fichier I get ~550 Mbit/s using one connection.