Many thanks, @rockstream .
I switched to NFS as suggested by you. Not so easy with my old NAS, but in the end I succeeded. Just for history and hoping that it may help somebody else i report the steps I needed to follow to get my existing SMB share working as NFS share as well.
In fact, web UI of my old NAS let me only create new shares as NFS ones. No way from there to make an already existing SMB share as NFS as well. So, I opened an ssh shell on it and run the following commands:
su -l root
cd /etc/
cp exports exports.bak
cat exports
echo "/path/to/share 192.168.x.0/24(rw,sync,no_subtree_check,wdelay,no_root_squash) #" >> exports
cat exports
exportfs -a
showmount -e
showmount --all
On a new trueNAS ssh shell I was able to mount&rsync as usual:
sudo mkdir -p /mnt/nfs_share
sudo mount -t nfs 192.168.x.y:path/to/share /mnt/nfs_share
df -h
sudo rsync -rtvv --progress --stats -h /mnt/nfs_share/ /mnt/streaming-pool/Media/video
Transfer rate is about 95MB/s, which is acceptable now. Tomorrow morning it will be finished at this transfer rate. Yes, I might create a cron job to let my PC be turned off and keep the job on the NAS itself, but it’s late and I have already learnt many things today 
Many thanks for your help, you saved me