I guess I should have clarified. The extension cord is used to bring power to the surge bar, which distributes it to the modem and router. When the surge bar used to connect the router and modem is plugged in via this orange extension cord, the connection slows to stupid slow. When I unplug the surge bar and replace the extension cord with a different one, now powering the equipment with a different cord, I'm back to my usual super ludicrous speed. It doesn't matter how far away the slack cord is lying. It's obviously a bad extension cord causing the problem, but why? Using a different cord, I'm even getting more bandwidth than my ISP advertises.

Yes, I know I've already solved the problem, but I haven't answered the question. Why is this happening? Hopefully an answer to this question will help me sometime later setting up a different network.