Hey there, I'm having the same issue with the same router and firmware version 1.2.5 Build 20190411 rel.52981(5553). I had the same issue on firmware 1.2.2 also. We should be getting around 200 Mbps download speeds, and connecting the modem directly to the PC bypassing the router entirely confirms speedtests around 250 Mbps. However, the router is capping the speed at about 79 Mbps. I've run the test about 18 times over 3 days now across 4 devices, some of which are connected wirelessly, and some via ethernet. Resetting the router to default settings fixed the issue for about a half hour, then it came back. Any help would be appreciated.

@Kevin_Z 

 

Resetting the router to stock settings appears to have fixed the issue for now, I will do all of this and report back if the issue returns. 

I am on the latest firmware. I have had my AC500X for about a month. It has been connected and working fine via ethernet and wifi up until I upgraded to 5G home broadband connection. Consistently, I am getting between 350 - 450+MBps. All my devices connect to the 5400.  

 

However, since my upgrade from NBN (max 72MBps) to 5G (450MBps+) I have noticed from day one that the router will work for 6 - 24 hours without an issue then all of a sudden the speeds drop and remain in the 30MBps range. Some times I have to restart it multiple times for it to pick up again and then it will be fine for the same amount of time then drop again. 

 

I know it is not my internet connection because I perform two speed tests at the same time, one of the 5G Gateway which I have never seen the speed drop below 300 and the other on the AC5400 which is connected to the 5G gateway via ethernet and that is in the 30MBps range. 

 

There is seriously something wrong with this device considering it is about a month old and it seems that too many people are having the same issue. 

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...