This is definitely not a reliable test. You're doing a speed test, over WiFi on an iPhone 6s. There are so many factors that could contribute to slower speeds including frequency, interference, range, and devices connected (If they went from 802.11n AES+TKIP it can be bottlenecked to 54Mbps.) Also if they're on 802.11b/g/n compatibility mode and a 802.11g device is connected it can also bottleneck the device. I'm sure there are issues with Comcast but this blog post identifies none.