Hello @vapol69
Sorry for the late update,
1. Please try a different Ethernet cable between AV600, better to use a new or better quality Ethernet cable which is cat5 above which supports 100Mbps or 1Gbp.
2. Try a different wall socket for main AV600, and AV600 extender
Is the PC wired to AV600 extender or wired to AV600?Did the PC get normal speed before?
If wire the same PC to router using the same cable how much speed it could get and what is the Wired Link speed at that time, what is the speed & Link speed when wire to AV600
How to check the associated/link speed on a wired client
Try to download tpPLC Utlity and check what is the powerline rate between 2 powerline units: Explanation about the Powerline Rate and Actual speed of the Powerline Adapters