When others are using your Internet feed, you get less. You could try a faster set of powerline modules next time or just get an Ethernet cable run to your area.
Hey there, for my family the internet speed connection doesn't get below 20+ Mb/sec (they are connected wirelessly (WIFI)), however, I'm conneced through electricity (using 2 powerlines) and whenever my family are connected to the internet, it gets below 5 Mb/sec for me although it doesn't affect them.
Despite that, at night when they are sleeping, it get to over 10 Mb/sec for me.
Any help would be appreciated.