Hi @Nemfoglalt,

Did you use the PC to test the speed of ISP modem when the speed on router turns down? 

In order to make a more accurate comparison, it is recommended that you connect your PC to the ISP modem for more than half an hour to see if the speed also drops suddenly?

Here are some tips which might improve the speed of your Archer Router:

    1) If Ethernet speed is slower than expected, please try different Ethernet cables (CAT5E or above) on the Router.

    2) Disable QoS, please access http://tplinkwifi.net and go to Advanced >> QoS to disable it.

    3) Disable Parental Control, please go to Advanced >> Parental Control to disable it.

    4) If wireless speed is slower than expected, please go to Advanced >> Wireless Settings to change the 5GHz Wi-Fi channel, you may try different channels to find the best one for your environment.

    5) Disable SIP ALG and UPNP, please go to Advanced >> NAT Forwarding >> UPNP to disable UPNP, go to Advance >> Security to disable SIP ALG.

    6) Try a few other speedtest servers, other servers might reach better speeds.