PowerLine networking (like 802.11) is a shared media network, such things would be dead if they were not so damned convenient. There is no great way of predicting the performance of any shared media technology without doing a lot of work before deployment.
In particular with PowerLine the condition of your power infrastructure is paramount. Unfortunately there is not much you can do to improve that, as if you were in a position to alter that infrastructure, throwing some UTP in the wall would have been you first pick (just don't place it next to those dirty power lines of yours).
For starters, don't test across 2 bad technologies. Test point to point over PowerLine, the wireless is only hurting your test.
Try different plugs, turn off major appliances, try different rooms (and circuits). Try it with and without your switch in place before the laptop.