Internet speed dropped tremendously in the last few weeks

First. Please try starting here if you wish;

https://forums.xfinity.com/conversations/your-home-network/connection-troubleshooting-tips/602da777c5375f08cdea3db9 

@user_la7m28 I am very sorry to hear you are experiencing slow speeds. We would be happy to look into the account and run some tests to see what is happening. Please send us a direct message with your full name and service address to get started. 

To send a "Direct Message" to Xfinity Support: 
Click "Sign In" if necessary 
Click the "Direct Messaging" icon or  https://forums.xfinity.com/direct-messaging 
Click the "New message" (pencil and paper) icon 
The "To:" line prompts you to "Type the name of a person". Instead, type "Xfinity Support" there 
- As you are typing a drop-down list appears. Select "Xfinity Support" from that list 
- An "Xfinity Support" graphic replaces the "To:" line 
Type your message in the text area near the bottom of the window 
Press Enter to send it. 

@user_la7m28 

Did you have a chance to see if any of those tips apply?