After a modem replacement we have experienced 9Mbps max download speed out of a 300Mbps package. All of this occured after a modem replacement to attempt to fix cable which was needless. This has been occuring for over a month in which the highest I have recorded the speeds to be at is 120Mbps. The speeds are the same for Ethernet with no increase or decrease.
DId you try any of the troubleshooting steps in Internet Troubleshooting Tips ? We don't even know what equipment you have and what you've done to troubleshoot anything.
I apologize I have done all troubleshooting in the book replaced cables, modem\router. This issue has been going on for over a month found out it is damaged cables outside. This issue has been brought to Xfinity's attention and was never fixed because no one scheduled an appointment to fix the cable lines for the neighborhood.