Some tips for improving WiFi performance;
There are other options. Google for WiFi repeaters / range extenders, WiFi mesh systems / pods, powerline ethernet adapters, MoCA adapters. Good luck !
I have having terrible wifi issues on this same model brand new modem as well. Constant loss over and over again even a few feet away. Xfinity says delivery of signal to modem is OK, but from modem/router to wireless devices is the problem. Not resolved, I have an active thread today at https://forums.xfinity.com/t5/Your-Home-Network/Constant-Wifi-Drops-CGM4140COM-firewall-logs-show-IP... . Under Troubleshooting/Logs the Firewall log shows Input Drops and Forward Drops, now continuing even with the Firewall disabled completely by a Tech. Out of curiousity have you checked the logs? No solution so far and Comcast suggests I pay for extenders. I think I will have to buy my own modem/router vs this rented one that clearly has issues. My old AT&T modem/router in the same spot in the house had zero issues.
I'd be happy to look into your speed concerns. Appreciate you taking the time to post on the forums. Please send me a private message with your full name for assistance.
wfi tops out at 53mbs for g and 300ish for N so if you getting less than that and you are wireless thats actually normal just change your billing or plug in directly (used to be a networking tech)