It may be best to modify the channel to see if that improves the performance. Generally it is recommended in highly populated areas like apartments, but it may solve your issue as well.

Try selecting a specific channel for both bands. For the 2.4Ghz, try channel 1, 6, or 11, and for the 5Ghz-1 band try 36, 40, 44 or 48, and for 5Ghz-2 try 149, 153, 157, 161, 165. That should get you the speed you are looking for. The width aspect of the wireless settings can be left alone.

Tony,

You might've missed it in the original post, but all of my PCs are hardwired so it's not a WiFi congestion issue. Also, as stated machine-to-machine transfers are close to 1Gbps (measured by iPerf).

Hello,

I have tried to upgrade my firmware from both the GUI and a direct download and I'm still having the same issues. After a reboot of the router, my speed tests are in a normal range for my gigbit internet speeds, 850-950mbps, but after some time it all slows down to about half that, 450-500mbps. Something is not right. I think it has to do with all the Parental features/QoS etc, but i don't have any of them turned on. I think its a bug in the software.

@NowhereMan73 If the solution dpak mentioned is still not resolving your issue, please send a email to: ussupportteam@tp-link.com

They will work with you further on your problem.

Any updates on this one?  I just had Spectrum Gig internet installed, and also have the C5400X.  Tech's equipment shows 960MB at the modem.  Router has steadily been reporting around 600MB, but my hardwired clients are reporting right around 250-300MB (plugged directly to router with  5ft of Cat6).  Something doesn't seem right.  Reboot doesn't help.    

@Brother_Bluto disable QoS and try a speedtest from a wired client.

Enabling QoS 'sometimes' cuts the bandwidth in half (bug?).

I'm also noticing this exact same issue - Wired PC's only getting ~350Mbps on a speed test, but Router speedtest showing 895down/795up.

There is no way to "turn of" QOS on the C5400x that I can see, and I just read the manual "just in case". 

This router is replacing a Netgear R8000P that puked and the VPN was never stable - I work from home and actually "need" every ounce of bandwidth I can get for my job.

Only getting 1/3 of the available IS NOT going to fly. What _should_ take me ~1minute to do, takes 3. When you have to do that dozens of times in a day, it adds up to real time wasted.

I can return the router up until 6/15/19, and I just might do that if there is no way to get around this issue permanently. I MUST be able to get 800+ on the wired LAN

Under "Advanced/Qos/Device Priority", I have my work PC set to Priority enabled, Timing = Always. This didnt make an ounce of difference.

- oh, and just for kicks, I just did a router reboot, while things are "better" I'm still only able to get 779down/724up on the PC. 

I've had enough unstable routers that "needed" to be rebooted "daily/weekly/whatever" that I'm not interested in owning one of them again.

I'll keep searching until I find a vendor that offers a STABLE product. Maybe I'll have to upgrade to something more aligned with commercial products.. 

Im having a slow internet since the last week because of this router, I would like to see a new firmware with improved QoS and bandwidth control (by device or ip range), the team should be worried about the product quality...