Thanks for the reply and suggestion.
I just tried disabling the 2.4 GHz band channel bonding. When connected to the Comcast router I get about 15 Mbps download speeds.
I also tried connecting to my Cradlepoint router (sitting right next to the Comcast router) and get about 50 Mbps download speeds with channel bonding disabled and about 74 Mbps with channel bonding enabled.
My routers are located in a private residence in a rural area with no other Wi-Fi signals nearby.