Slow Internet Speed with TC Gen 5 and Time Warner Modem

I am using the 3TB Gen 5 Time Capsule and Time Warner Cable Modem. Recently TWC increased the internet download speed to 300 Mbps. Using the TWC Speed Test I am getting around 95 not only when using TWC Wi-Fi but also when my iMac is connected to the TC directly over ethernet. I do get close to 300 Mbps when TWC Modem is connected directly to iMac over ethernet. I'm surprised that the WI-Fi speed is around one third, but more surprised that the speed is also one-third when connected over ethernet. I don't understand how the TC and TWC modem work together. My use of the TC Wi-Fi rather then TWC Modem Wi-Fi was recommended by Apple Support when they helped me set up the TC about six months ago. I'm not sure what I should be expecting. TWC replaced the cable Modem so that isn't the culprit. TWC also said to achieve the 300 speed the Modem needs to be connected directly over ethernet to the computer. There may be a configuration issue or lack of understanding on my part. Any help in sorting this out would be appreciated.