Slow cabled speed on NightHawk x6 R8000

Hi fellow forum members, 

 

I own a X6 R8000 which i recently upgraded to firmware V1.0.4.28_10.1.54

Since that moment, i notice very slow cabled speeds to my desktop computers (wifi only 58 down, 17 up)).

I have no parental controls enabled, 

I have no Dynamic QoS enabled,

no VPN, no fancy other things, ....

I get an internet speed of approx 230 down and 30 up. 

When cabled desktop is running speedtest, I'm not getting over the 100 down.

The whole network is Gigabit (router, switch and computers).

Can anyone help me get more out of my router?

Thanks in advance.