In a PC connected via the Powerline plugs, go into the Network Connections folder ("Start > Run", type ncpa.cpl and click OK) and double click the entry for the Local Area Connection. This will display its Status - what "Speed" does that show?
With a direct connection to the router this would normally be 100Mbps. Via the Powerline plugs, it may be less but, if it's ridiculously slow, then there is a problem with the network. It has nothing to do with the ISP or MTU settings, that is the internet side of things, you problem is in the LAN.