Can't say if it is defective or not but I had a similar situation after I first got my C5400 v2. I would try the following (assuming you haven't already done so) and see if it helps. . . .

1. ensure that you have the latest firmware available for your region. If not, upgrade.
2. reboot the modem and then the router (keep the modem connected to the WAN port during the reboots).

In my case, after doing the above I managed to get all the speed from the router that my ISP was delivering and everything is still working well. Good luck . . .

Tried that also with anything else disconnected from modem. No improvement.

Had the same issue at first. I was able to isolate the problem to the NAT Boost being deactivated when QoS is active. Go to QoS Device Priority and remove all devices from the priority list. Go to QoS Application Priority and select Custom and leave every slider on Normal. Hope this works for you.

sca wrote

Model : C5400

Hardware Version : v2

Firmware Version : 20170912

ISP :

I have a gigabit internet connection. Speedtest gives mee 900+ Mbps when wired directly to my modem. Tested with laptop and Apple TV 4K.
A brand new Archer C5400 v2 only provides half the speed - with the same devices wired to it. Cat 6 cables in all tests.
Speedtest from the "Thether" app also shows connection speeds around 4-500 Mbps both up and down - I guess this should test the speed between router and modem.

So - is my Archer a defect unit, or are there fixes for this?

Regards,
Steinar Carlsen

No, it's not defect. I have Xfinity Gig internet with my own modem & C5400. Most of the time I get 940/41. You should consider a factory reset of your router or do a restore from backup, I know it's a pain in the butt for a factory reset. For whatever reason, every week or two my download drops to half of what I should be getting. Unplugging everything and restarting does not get me my speed back. What gets me my speed back is a restore from backup or just a plain old factory reset. I'm doing this on a weekly basis, which is pissing me off. Other than that, my c5400 provides excellent connections both wifi and ethernet.
P.S.
I dropped the Gig speed internet for Blast Pro 250/11 ........ Gigabit was overkill for me, didn't really see a big difference. My equipment is pushing out 290 down which suits my household well.

I would consider the need for weekly factory resets a defect.

I am currently testing my second c5400 router - which has exactly the same problem with WAN-LAN throughput. Version 2.0 and latest official firmware.
So this appears to be a half-gigabit router, not a gigabit router.

Is this the new router or was it working properly?
Latest fimrware and factroy reset but still the same speed?
When it's connecting to modem via a cable on the same device, it's 900+Mbps but it's 4-500+Mbps when it's connecitng to Archer C5400 V2?
Make sure there is only one device which is connecting to the router and there is no other application running when you test speed.

None of the two units I tested were able to achieve a decent WAN-LAN connection speed. I now have returned them both, and will try another brand wich is not limited to being a half-gigabit router.
Both were version 2.0 with the latest firmware. I never tried a factory reset - but that should not be neccessary. The first was updated to latest firmware, the second already had the latest.

Steinar

I swapped the TP-Link for a LINKSYS ea9500. This one tested OK with full gigabit speed on wired connections.

I had this exact same issue. Modifying the QoS settings to be the default/balanced setting (as jcgallagher stated above) made everything perform as expected.