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When saving torrent files from a Win10PC over a 1G LAN to a Network Attached Storage (NAS) I get unusable slow speeds (just a file transfer is very fast).
Speaking about the issue with Synology the mfr of the NAS i get this response:

"Since just regular file transfer to the NAS through a mapped drive are operating at normal speeds, it is likely a mechanism with the torrent that is the cause of slower download speeds. It is possible that it is opening multiple connections to the drive causing it to slow down."

Is there a way to limit the number of network connections to the NAS to experiment for speed improvements over the LAN?

I haven't had this issue exactly, but I prefer to download to a local disk, then use a script file to get windows to move the file for me using Robocopy. I've found it to be more reliable when dealing with folder transfers etc.

You might have more luck using that method instead.

Here is the script I use:
Robocopy "E:\Downloads\To Transfer" Z: /s /mov

You'll need to customize it for yourself. In order to do that simply replace the following marked in bold.
Robocopy "E:\Downloads" Z: /s /mov

The first part in bold is the location of your finished download, the second is the location where you want it to go.

Put this into a notepad and save it as MyScript.bat or whatever you like. Be sure to change the 'Save as type' to 'All Files' for it to save as a script. Then add it to qbittorrent. Then once a new torrent finishes, it will run the script invisibly and move the file to your destination.

If you use this method it will move the containing folder as well as it's contents to where ever you want. Currently I download to an external USB HDD, then once complete, run some other scripts to clean things up then use the code above to transfer to a network drive.

As Robocopy is using the Move command it will act as cut and paste, meaning, once moved it will only exist where you moved it to. You may notice it will leave an empty folder behind in the original location. You can just delete that.

I would recommend playing around and testing it 1st as it may take a little while to transfer. I've not really tested for speed. The only way you can see it has complete is by seeing that the containing folder in the original location is empty. It the files are still visible, the transfer is still in progress.

Use at your own risk.