My qnap device slows down the internet like crazy, to diagnose this its simple, when internet slows down to like 1mb speed i simply either power down my qnap or unplug my Ethernet cable and the speed shoots up like it is normally. Start up the qnap or plug in the cable and boom im back to a slow line.
There are three possibilities and 3 solutions;
1 – Shut Down Download Station;
1. If you have been using the “Download Station” to download public BitTorrent files, you should be aware of how BitTorrent works – after your download is complete, you become a “seeder” and others download from you (unless you stop it). Seeding a handful of popular public torrents can easily saturate a home internet connection. This is not too bad, things are working as designed. All you have to do is stop the offending torrents.
2 – Update NAS & Install Malware Remover;
2. Your QNAP could be compromised (hacked, pwned, etc.). It could be part of a botnet participating in DDoS attacks, serving illegal content, or who knows what. You mention that you updated the firmware after this started – how long out of date was it before? You also mention it is exposed to the internet through the QNAP cloud features, so this is a realistic possibility.
This is very bad. A system compromise can be pretty well disguised and hard to confirm or get rid of. If your ISP has some bandwidth usage monitoring tools, you can see if they show an unexplained increase since this started.
3 – Change your NAS IP adres & Modem port on your Router;
3. You have a faulty network interface, which is killing your local network. This is rare, but I have seen it a few times. It is better than #2, but not too great either, as you will likely need a new NAS. If you have a network of several devices, you can see if the communication between them is also affected by the QNAP (e.g. file sharing between two computers slows down a lot). A managed (or at least semi-smart) network switch may be able to tell you something, if you have one.
More info: https://forum.qnap.com/viewtopic.php?t=124322