@MartiX 

First the AV1300 is the maximum powerline link rate it can get in ideal network environment, while when we put it in use, the noise of the powerline network and interference will affect the performance of the adapter. Even though they are plugged into the double socket, the interference still exists. 

And the TL-WPA8630PKIT has the gigabit ethernet port, and the current link speed is good which can satisfy the daily network activities. 

May it help. 

@MartiX 

By unplug the electrical appliance, we can reduce the powerline noise from these devices. You can give it a go and verify whether the speed gets better. 

While we cannot say that there won't be interference anymore. Cause the power line is invisible (hidden by the wall) mostly, it is hard to say that it is total clear. 

May it help.