@SlavaGo 

I am having the exact same issue after upgrading my Archer C5400 to version 1.2.5 Build 20190411 rel.52981. I have a gigabit fiber connection and noticed massive slowdowns. When the router is freshly rebooted download speeds are fine (around 100 Mbyte/s) but after a few hours it slows down to 30 ish Mbyte/s. A reboot of the router fixes it, but after a few hours the connection speeds take a real dive.

This is obviously a very annoying and unwanted bug.

@Anthrow Same problem here, I live in Brazil and have a 300Mbp/s plan directly connected to AC5400V2, which worked fine before upgraded to 1.2.5. I had to rollback to 1.2.2.... Same on TPLINK !!

So what about a fix @Support ?

Exactly the same speed issue! I'm just furious about it! I live in Russia (Archer C5400 V2), the tariff is 1 Gigabit - the speed will also be reduced to 200 megabits. Only rebooting the router helps! Rolled back so far to the previous firmware. But something tells me that we will not see the corrected version of the software soon. Apparently in the future I will prefer equipment from another manufacturer.

@Anthrow Exactly the same problem with the latest firmware update! It is annoying that TP-LINK does not pay any attention to the problem that it created for its users who purchased their premium router models. The company did not even recall this update, and users act as beta testers for raw and erroneous updates!

@Kevin_Z Thanks for the answer!  Yes, I would like to receive a beta of firmware with corrections (s.miklukho@mail.ru).  And now for the points that I have now.  Hardware Version: C5400 V2.  It is connected via LAN to the optical ONT of the MGTS provider with a gigabit rate.  Measurement speed 930 Megabits via Ugreen Cat7 cable.  Configured 2 access points 2.4 and 5 GHz.  The speed at 5 GHz according to measurements is about 600 Megabits.  After a few hours, I find that the speed over Wi-Fi and LAN drops to about 220-250 megabits and I have to reboot the router, after which the problem disappears for several hours.  I use the default settings, I do not use any additional ones.  After a drop in speed on my router, on the provider's optical terminal, the speed corresponds to a gigabit rate.  For the sake of experimentation, I installed a TP-LINK gigabit switch, at the operator’s output there is always a stable speed equal to gigabit, but the C5400 V2 router uniquely behaves inappropriately on the latest firmware.  It is very sad that buying such an expensive top-end router as a result, you get so many problems with it.

@Kevin_Z I want to test it too, please... my email is riazigi@gmail.com

@Mtecharena after reboot problem solved for few days and than again. So internet provider was not relevant in my case and after that TP-Link support explained me that once you enable QoS and antivirus, it can't be dsiabled (internaly) you can turn-off but it still running somehow and sugested me to install custom firmware that entirely disable QoS and antivirus I decied to revert to previously working firmware and not to play with TP-Link anymore firmware as this is russian rulete or even worse, every new firmware is worse the prevous one , so why bother ? It works, don't have any security volnurabilities (known) I'm not upgrading :). And not buying TP-Link anymore, good hardware, bad soft and very very bad support , I spent about 1 week anwring them that it's wired connection, I don't need to put laptop near router, that after reboot it works well for some time and lasyt answer from them was that 1.2.5 is some custom version, as they don't see it in the web site.