Like most have said, the problem is due to an improper ground. The router etc has a ground reference that it gets from the extension cord and if the extension cord is not grounded properly then you have the introduction of noise.

Noise is a big problem in audio devices for the most part and is very troublesome in transmission devices. Each device has it own noise floor and hence the device supply the source of power to the device in question should have a lower noise floor, in order words the extension cord should be less noisy. The more noise that's introduce by the extension cord the slower your connection will be, since this noise is fed into the flow path of packets being sent to your computer and back from your computer. This transmission collides with the noise causing a bouncing effect instead of the normal straight flow with direct contact with the sidewalls like in a normal optical fiber. Since there will be more collisions than normal this slows down the rate of transmission and hence your speed is slowed down tremendously.

Noise it the key element that slows down transmission followed by Harmonic distortion and indeed an improper grounded wire can introduce noise, since that introduces a slight open circuit leading to a partial incomplete circuit.

I hope I didn't confuse anybody and I hope this helps a bit.

Guidance...