Before Robert B. jumps on me, I guess I better elaborate on that statement.
Slow response can indeed be attributed to congestion within the cable network itself. Furthermore, it can be caused by ingress/egress; e.g., "signal leakage." If there's a break in the physical shield (wind-induced crack; hungry squirrel; drunk driver; errant backhoe; vandalism; whatever), ambient RF from the outside world can enter the cable and interfere with cable signals. Such interference can severely degrade digital signals.
Pinpointing the source of a slow-response problem lies in an analysis of the physical locations of the complaints. If all complaints come from one node, then that node, or one of the cables or fibers connected to it, is clearly the problem. If a node is simply overloaded (too many subscribers hooked up), the node can be split into two or more smaller nodes (equipment sales guys call this "scalability").
But if complaints originate across the entire cable network, then the problem can usually be attributed to the headend equipment, or whatever's upstream. Unless it's an extremely small cable system, with only one node, this is a useful tool for isolating slow-response problems.
Computer problems can cause response problems too. A computer owner who switches from a dialup connection to a cable modem (or DSL) expects faster response. But a sluggish computer connected to dialup is still a sluggish computer when connected to a high-speed connection.