Speed problems are most often due to poor connections between your modem and Comcast's network, or a weak or poor quality Wifi signal. What is the model number of your modem? Are you using Wifi or Ethernet?
If you want to troubleshoot this yourself, please see Connection Troubleshooting Tips. If you still need help, please provide Information Requested for Connection-Related Posts. If you are using Wifi it's best to switch to an Ethernet cable connection if possible while measuring speed and checking the reliability of your connection to Comcast.
If you can't find the problem or you'd rather have Comcast do the troubleshooting, call them at the phone number on your bill or 1-800-Comcast, or chat with them using the Internet Support option at https://support.xfinity.com/chat. If they can't fix the problem remotely, insist they send a tech out to identify the cause and correct it.
If the tech finds bad coax, splitters, amplifiers, or connections in your home (even if Comcast originally supplied them) you'll probably have to pay for the visit unless you have or sign up for their Service Protection Plan (https://www.xfinity.com/support/articles/service-protection-plan, about $6/mo) and keep it for at least 60 days. If the trouble is due to a faulty Comcast modem, eMTA, gateway device, or anything outside your home, you shouldn't be charged.
Since the repeal of net neutrality Xfinity has 100% been throttling speeds for certain sites. Prior to the repeal, my download speeds for certain sites were almost always between 2 mb/s and 5 mb/s. Now during the day, the speeds hover around 90-100 kb/s. After about 11PM, the speeds get up to about 1 mb/s. It's absolutely ridiculous and it's definitely a sign that Xfinity cares very little about consumers (hence the removal of the pledge).
What modem do you have?