@Retrography 

Thank you very much for your kind reply.

Could you please provide me a picture of your network map and please also mark out which Deco the NAS is connected to?

Did you access the NAS device via its LAN address, or domain name/WAN IP address?

Can you improve the transfer speed every time when the speed test is running?

Thanks a lot and wait for your reply.

@TP-Link Here is the network diagram:

I have experienced the slowdown on two devices (TV and Notebook), both connected to the main Deco, sitting between one and three meters away from the Deco. The NAS is also connected to the main Deco, but via an unmanaged gigabit ethernet switch 

The issue has occurred several times, but the timing is pretty random and often hits when streaming to the TV. Restarting the NAS doesn't help. Restarting the main Deco does help. I have observed this on both TV and Notebook. Running an internet speed test on Deco also seems to solve the issue temporarily, this I have tested this on the Notebook several times. Since I don't know how to reproduce the slowdown, I can't retest the internet speed solution on another device (that is, on the TV). But I remember having the same issue on the TV using a wired connection. Meanwhile, streaming from the internet seems to work fine, or at least the slowdown is subtle enough not to be noticed.

When I access the NAS I normally use the mDNS address (mynas.local), but I can't be sure I have done this every time. I have used a SAMBA share on the Notebook, while the TV transfers the data either using Plex or DLNA.

I also have to note that enabling the internet security features (intrusion prevention, etc) results in the same type of slowdown within the LAN, and then nothing helps until I disable all the options (I don't know exactly which option creates the issue, Malicious Content Filter or Intrusion Prevention System).

@Retrography 

Thank you very much and I would like to forward your case by email and please have a check of your email box later.

By the way, there is a new firmware for Deco X60 V2: 

Deco X60(EU)_V2_1.5.1_20210204

https://static.tp-link.com/2021/202103/20210311/Deco_X60_V2_1.5.1_20210204.zip 

Could you please try to download the firmware and log into the web interface and manually upload the latest firmware?

Thanks a lot.

@TP-Link Thank you I receive the email.

I have also installed the new firmware, and for now I haven't experienced any slowdown with the new firmware yet (since yesterday). I will keep testing for a week and report back.

@Retrography 

Thank you very much and please feel free to update here once the issue started again.

@TP-Link So, the issue did occur again yesterday, once more while streaming to the TV. I tried to internet speed test trick, and it worked again! Please let me know how we shall proceed to resolve the issue. 

@Retrography 

Thank you very much for your update.

In this way, I have forward your case to the senior engineers for further assistance, and please feel free to update the email.

Just for the record, in case anyone else is experiencing wifi / ethernet slowdown issues with Decos (between your computers / nodes in your network):

The issue (at least in my case) has to do with Trend Micro Homecare (Antivirus) service. Disabling it often resolves the issue. The only problem is that the Deco app interface does not reliably work, and may leave the service on while reporting it is off!

If you have turned all the three Homecare services off and you are still experiencing this issue, you may have to telnet into your main Deco (port 23, no user/pass) and stop the `tm_shn` daemon. 

If this does not work, simply kill all the instances of the `tm_shn` process. This latter solution will probably interfere not only with Homecare, but also with QoS and Parental Control. That said, it will undeniably resolve your issue as well...

I know for sure that up to firmware 1.5.1 Build 20210303 Rel. 31993 this issue remains unresolved. Hopefully it will be addressed in the future upgrades.