@Milobar I had just half of the speed at the beginning. Resetting to the factory default settings did the job. I am running at full speed now.

QoS setting causes a significant loss of speed. If I turn it off, then I see a full speed. If I turn QoS on then, I have 2/3 of speed.

@Dalibor 

Woah, glad I'm not alone! 

Within the past year, I bought a C5400 router... same behavior as described in this tread. Speed is consisent for a few months, and then bang... it drops. I have a 400mbps connection and it drops to 90. The only fix was to do a factory reset every week.  Bough a second C5400 and returned the other one. The new one did the same EXACT thing after a few months.

I decided to cough up the money and upgraded to the C5400x. Now, exactly 2 months after working without any issue, the speed drops!  Factory reset is the only solution, but it barely lasts a few hours and the speed drops again! It almost seems like they are programmed to work as such! Why am I ok for months and then it starts acting up... this has been the case with 3 TP-Link routers?!

I installed the beta firmware provided within this thread. Can't really confirm it works for the moment because by simply rebooting the router, full speed is restored. I'll know in a few hours or days. 

I guess I have the return this one again and move to another brand and if it's still dropping.

@rico79 just  reporting back - Beta firmware did not cut it for me. Speed was restored for about 3 hours, and dropped back at 90mpbs.  

@Kevin_Z 

same i have a Gigabit internet i always restart the router to get a full speed 

@Alexlofern Even that didn't work for me on a c5400x - I had to disable Application Priority completely...

Every 3 or 4 days my C1200 internet speed drops from 100mbps to 12mbps download. I turn it off and wait 5 minutes and it comes back.

If I turn it off and then on again straight away, it doesn't fix it. You have to wait 5 minutes.

Resetting the router fixes the problem also.

This problem only started happening about 6 months after purchase.

same issue here with the latest firmware 

back it to the older version (firmware) seem to be working for me no issue so far

@Paul_Tan Is there already a fix for this issue? as it is very annoying. I also contacted my ISP before, but they could not see any issue.
And then i found this thread. I installed the BETA firmware but it still did not work. My internet connection speed is lowered by 50 to 75%

@Milobar 

I have the exact issue on my C5400 v2 as everyone else.  The drop seem to happy in hours after the restart.  I have tried to disable QoS and AV but it stays slow.  Wired ports have no issue what so ever.  After 2years, I am thinking to upgrade to the latest AX11000 but seeing this trend even on AX50 is concerning.  I might have to switch to other brand sorry if TPlink is not going to fix this problem.  I can see many unhappy customers here and it will be soon many more if no one in TPlink is taking this seriously.  Sad.

@Milobar i got same problem. Loaded with their beta firmware but still the same. Any solution from u?