TCI offered the explanation that the system was still new in my neighborhood. As the system became familiar with our home's usage patterns, its speed increased. DSL does not suffer from the shared-resources pitfall. Speed levels are set by how much users are willing to pay. DSL has its own set of limitations.
Costs escalate dramatically as rated throughput increases.
The minimum rate, 256Kbps, is well under the advertised throughput of a cable modem but costs more. Then there is the Internet itself, as well as speed differences in browser software and computer performance, including graphics. A slow Internet server will bring any connection to a crawl. Your computer's ability to handle plug-ins, complex layouts, video and sound also will affect your sense of speed. After working with both systems, I concluded that at a certain level, fast is fast.