[chipotle_coyote]

Actually, what you're paying for is probably "bursts up to 100 mb/s" rather than "sustained traffic of 100 mb/s," whether or not it's marketed that way. ISPs commonly "oversubscribe" trunk lines dramatically. Back when I did trunk management at a big ISP in the dark ages of the internet (the late '90s), we oversubscribed at about a 5:1 ratio: basically, we sold 5 times as much bandwidth as the trunk actually had. As long as there was free bandwidth available you could get your full amount, but the amount you were guaranteed was one-fifth of that.

Of course, those were business lines and all those numbers were actually in the fine print of the contract. As far as I've been able to determine, if your residential ISP is only oversubscribing at a 10:1 ratio, you're pretty lucky (I've seen some reports from industry consulting firms that suggest 50:1 is more common), and the chances are they're not guaranteeing a minimum speed they can be held to.

[anticensor]

There is a minimum speed guaranteed for most home subscribers but that is a fraction of dialup speed. Home routers do latency profiling to have a better connection stability; meaning even if you do not fill bandwith, all your requests will be delayed to smooth out until the buffer completely fills.

In most operators, limited bandwith users are oversubscribed and unlimited bandwith users are linked to dedicated channels. Oversubscription ratio is around 20:1 for DSL and 100:1 for mobile here in Turkey.