OT (but related): I pay for a 20Mbps connection in the DC area through a cable provider. On a number of speed-test sites, I only get around 6Mbps. I tried calling tech-support, but my representative went down a script asking me about clearing my cookies, etc. I was unable to convey to him that my speed wasn't "slow", it was just less than the advertised rate.

Is it normal to get far less than the advertised speed for a residential cable connection?