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Hello,
I have WAN 800 Mbps and when downloading torrents to the local drive on Windows machine (W10) download speed is in the range of 750 Mbps. However, if the download path is to NAS (SMB on Synology) the speed drops down to around 150 Mbps. There is nothing i could do to improve this so far.

There is no any LAN bandwidth issue between PC and NAS: if I copy downloaded file from PC to NAS it goes at around 900+ Mbps.

I other words, even if I first download file to PC and then copy it NAS, then (based on the total download time + copy time) the average speed would be much higher than if I would have downloaded it to NAS right away. Yes, it is a sort of workaround, but no need to say that it is not convenient and something is wrong here. And still it is very far from 700-750 Mbps I would expect for download directly to NAS.

I've checked this on 2 Windows machines, tried changing various settings (like pre-allocation etc.) nothing helps so far.

I think this issue is not noticeable for those who have WAN less than 100 Mbps, maybe that is one of reasons it is not yet a "known issue".

Any advice would be much appreciated. Thanks.

This has nothing to do with qbittorrent. The problem is SMB.
SMB was never meant to be used for IO intensive purposes. Try iscsi, you will get much better results. The problem is that any file downloaded via iscsi can only be visited from the windows machine, to NAS, the files are block level data that cannot be read.
Just install qbittorrent on your NAS, that is the true best solution.

Thank you very much for prompt response.

Strangely, copying the file to NAS is very fast, as I mentioned. BTW, in the traffic monitor of the PC I see a bit strange thing: upload curve (which means that file is being uploaded from PC to NAS) matches, as expected, the torrent download speed, but download curve shows approximately double speed vs actual download speed from the Internet. So that it looks like there is a large "phantom" overhead traffic to NAS (or something else, no idea what it can be).

Yes, agree - installing Docker/qBittorrent on Synology indeed would be the best solution. The only question then would be whether it would be possible to migrate all the torrents from Windows machine to Docker...

BTW, is there any kind of path variable (like %xxx%) to be used in Docker/qBittorrent corresponding to the path of the torrent being started, so that each download would be stored in the same location as the torrent file (without a need to write in the path manually and to change each "\" to "/")?

This is not a perfect analogy, but I think it will get the message through: you know when testing a SSD/HDD, there is sequential write/read speed and 4K random read/write speed? Copying large files is more like sequential read/write while downloading p2p is more like 4K random read/write. The phantom speed is possibly due to SMB's caching mechanism. Remember, SMB was never meant to be used for this.

I have never used windows version so I can't tell you how to migrate. But in docker setup, torrents are stored in separate BT_backup path, and downloaded files are not expected to be put in the same path. You can use the "copy torrent file to" option if you want torrents to be in the same path as the downloaded file.

Thanks for your response.