With a properly "tuned" wifi, your speeds should equal or exceed the 100mbps Spectrum "advertised" speed.
Cable companies generally over deliver, because in heavy use periods, things will tend to slow a bit due to neighbor's traffic. They try to average the advertised speed so in off times, you may get 10-20% faster speeds.
Time Capsule is a dual band router. Depending on which model TC, the 5Ghz frequency will be either 802.11n with speeds up to 450 Mbps, or 802.11ac with speeds up to 1.3Gbps. In both models, the 2.4Ghz speed will generally be no more than 150Mbps or so. But, most people configure Airports with many of the default settings. One setting is the SSiD on 2.4 and 5Ghz frequencies which is the same unless you change the 5Ghz SSID.
I tend to use a theme like NETWORK and NETWORK5G but use the same security\password. By joining NETWORK5G but "forgetting" NETWORK, you force devices to connect to the 5Ghz network.
2.4Ghz suffers from a number of factors that can slow it down, but has a longer range so often appears as the "strongest" signal and devices will connect to it. 2.4Ghz is the same frequency range of microwave ovens, radar, wireless home phones, wireless keyboard\mice, bluetooth. And because it has longer range and is ubiquitous, chances are your neighbor's wifi signals are in range and interfere with yours. 5Ghz has far fewer devices operating in that part of the spectrum, and thus much less interference, including from neighbors (shorter range).
If you separate the network names and connect to 5Ghz only, your speeds will more than likely approach or exceed Spectrum's advertised speeds.
To know if it is your modem slowing things down, connect something with ethernet to the TC and run a speed test. That will tell you wireline speeds, a good measure of what Charter is delivering and your modem and router are able to do. If that is 100Mbps +\-, then it is time to look at optimizing your WiFi.
