How fast is a cable modem ?
A typical residential cable modem connection ranges anywhere from 3 to 15 Mbps (Megabits per second) downstream, and 1-3 Mbps upstream.
Cable modem speed is not guaranteed and often fluctuates, since the available bandwidth is shared between many users on the same node. Cable modems are also typically capped at a particular speed, depending on your ISP. Some ISPs provide different tiers of service at a different price.
Also, note that the available bandwidth, or "speed" between users and the Internet provider is often much faster than the capacity of your ISP's backbones. In order for providers to offer competitive rates, they oversubscribe, and the fact that you have 15 Mbps to your ISP does not mean you can reach that speed across the Internet to a distant external server on a different network. This is true for all residential broadband connections.