This issue was driving me crazy for MONTHS.  So glad I found this forum.  Why would you put in a feature that clearly screws up the speed?  Just wow.  Turned all of this off and now no issues.  All wireless (streaming) devices were also having problems maintaining their streams.  

So what did you do that fixed so the speed in WIFI will always get full speed ?

i have the same problam ? so any company update respond ? and can the vactory replace for me  to new router ?

Same issue here. 

TP-Link Archer C5400X

Able to get gigabit Internet WAN speeds after rebooting the router. Wait a couple hours and the speed drops to less than half-gigabit (around 400 Mbps). Rebooting the router every few hours just to maintain gigabit speeds isn't an option.

Cat 6 Ethernet hardwire install from the router to the computer running Speedtest

Application Priority is set to "Normal" for the gaming, streaming, surfing, chatting and downloading options

Device Priority is normal for all devices

Firmware version: 1.1.0 Build 20180904 rel.41828(5553)

Hardware version: ArcherC5400X v1.0

This problem has been reported numerous times in this forum, so it doesn't look like an isolated event.

In summary, when the router Internet WAN speed maxes out at around 400 Mbps with gigabit Internet service, rebooting the router resolves the issue for a couple hours.

You can say I'm not a satisfied customer.

This is an old thread but here was my issue with the c5400.

ISP - Cox

Speed Tier - gigablast

It worked fine at first.  About a month in I noticed that the internet was starting to lag down.  Ran speed tests and was only getting about 300 mbs ar each computer attached to the router.  I did have a asus router in repeater mode in a seperate room as a wireless bridge for another wired device.  This was the setup I was running from day one.  I chatted with a tp link agent and multiple cox agents over the course of 6 months trying to figure out the issue.  The speeds kept dropping.  The entire time I was getting about 850 to 900 directly hooked to the modem.  I finally ended up replacing the c5400 with the asus router.  Speeds are coming through nicely.  

I honestly believe there is a bad batch of modems out there as I have a friend that has the exact same setup as I have and has had zero problems with speed...  I will not be using tplink products again due to poor quality and poor customer service.  

@sca 

@Kevin_Z could you post the link in this thread?