Internet speed not as advertised

Hi, I am an xfinity internet service user for a long time. I was on performance starter plan for quite a long time. Recently received a promotional call to upgrade the speed from 25 to 100 mbps for a smaller increase in bill. I grabbed the offer as it was offering no contract. Now when I check my internet speed I have never received more than 50 mbps. I believe xfinity is advertising wrong information. If anyone face similar issues please let me know if you have a solution. I am planning to raise a FCC complaint.