Is DSL dedicated, while Cable modems shared bandwidth ?
A common misconception is that residential DSL is dedicated bandwidth, while Cable modems provide shared medium.
This is only partly true - for the segment between you and the ISP's central office, and that is rarely the bottleneck of the connection. From the Central Office out to the Internet, both Cable and DSL share your ISP's backbones, whatever they are. Residential broadband is oversubscribed, whether cable or DSL - usually with 20+ times as many subscribers as the maximum backbone capacity. Since your ISP's backbones and peering arrangements are often the bottleneck of the connection, and it is shared medium, both residential DSL and Cable may experience slowdowns at peak times.
One can argue that DSL is dedicated between you and the Central Office (and shared from there on), while Cable is shared for that "last mile" segment of the connection as well. However, cable technology is able to push much higher bandwidth over that last mile to support multiple clients, and the signal does not deteriorate nearly as fast because of distance. Because of this, cable modem technology may be somewhat more prone to variations in speed than DSL, however, it usually offers higher average throughput.
Note: One can compare a speed test from their ISP's servers (local/nearby test) vs. a speed test from a distant location to determine whether the speed limitation is in the last mile.