I have the C5400 v2 with the latest stable firmware, 1.2.5 Build 20190411 rel.52981(5553).  I had to go into the QoS section of the Advanced tab on the home page of the router setting menu to manually edit the Total Bandwidth entry to 900 Mbps down and 20 Mbps up for my supposedly 600mbps Xfinity service.  At the wall of the house it was around 880 and at the modem it was around 680 or so.  Now, I get anywhere from 620 to 690.  No beta firmware required. 

There is a button on the Basic tab of the Network Map page named Speed Test.  When I tested, it would register no more than 315/12 and that is what showed up automatically in the Total Bandwidth entry in both the QoS  Application Priority page and Device Priority page.  When I manually edited the QoS Application Priority page entry to 900/20, the bandwidth restriction to both my wired and wireless devices went away.  It also automatically changed the entry in the QoS Device Priority page to match so that I didn't have to make duplicate entries. 

Hope this helps... 

@Kevin_Z is there any solution for same issue with c5400x v1 model?

@Saparski I'm not familiar with that model and do not know what the router settings menu looks like, but disabling QoS or raising its speed threshold should be worth a try. Always remember to save your settings and reboot your router to see whether any changes you make actually help... 

@mikengail, manual setting of the speed in QaS section realy helped at C5400x, thank you very much! 

I hope it will be fixed with new firmware... and I hope new firmware will be with mesh support for both models :)

@Saparski Yeah, I was getting only about 200 down with a 600mbps account. Raising those QoS limits got me well over 600 consistently.  Glad it helped you. You may have to set your router to automatically reboot at night, but it's still better than having it choke down half of your potential throughput all the time... 

Same problem here. I tried all tips listed in topic, but after a little time the speed is lowered again and only if reboot the router his back and the loop goes again.

@Ulisses I was having that problem too.  I had to download the beta firmware to fix the periodic rebooting requirement.  It's not available using the router's own menu interface.  You have to get it from tp-link's support page for this router.  It may appear to be the same file, but it's not.  The file that gets downloaded is named, ArcherC5400v2_EU-up-ver1-2-5-P2[20191204-rel63911]_signed_Beta.zip.  It was recommended by tp-link tech support and worked for me.  No more rebooting.  Hope this helps... 

@Kevin_Z Do you have any kind of update? I have the beta firmware and still only getting half or lower speeds. I tested the speed test going into the router and its getting up to 600mbs but im only getting 200 or lower from either wireless or ethernet vehind the router. I dont understand what is going on. Archer c5400 v2. This is a pain!

@Billtwa  Look at my post #65 above and try those settings in the firmware's menu...