Well, 256K probably refers to either 256KBytes/second maximum bandwidth or 256Kbits/second maximum bandwidth. 10Mbps == 10 Megabits/second = 1.25 MegaBYTES/second == 1280 KBytes/second.
(256Kbits/second = 32 KBytes/second, 8 bits in a byte). So the connection between your computer and the cable modem is up to 40 times faster, and at least 4 times faster, than the connection between your cable modem and the internet. This is normal.
Next, how are you judging how “fast” your connection is? A connection is a two-way street, the server has to have a big enough pipe to feed you data. boards.straightdope.com, for instance, will never send you anything faster than a few KB/second, and often won’t send you anything faster than a few bytes/second when it’s under heavy load.
So to test your connection speed, you should be using a website devoted to such tests, like http://www.dslreports.com/stest. (google for “internet speed tests” to find servers that are potentially closer to you). If your connection to other servers is slower than the speed reported by such sites indicate, then the bottleneck is the server you’re connecting to, and no amount of spare bandwidth on your connection is going to make the site load any faster.
Once you have real numbers, and you can be sure you really have a problem, you can start using tools like ‘tracert’ to tell where the bottleneck is.