Internet Speed is Half What I Pay For

Someone from a competing company knocked on my door and told me I am way overpaying for WiFi in my area. She showed me how to independently check my internet speed and I was shocked to see that my internet speed is consistently less than half of what I pay for on the Superfast plan. I work from home so this is a MAJOR issue and super upsetting to think my money has been going to waste all this time. I tried contacting Xfinity assistant but it’s useless, and every time I call the support line it turns me back to the assistant. I need HELP! I just want to get what I pay for or I’m going to have to switch to the other company because this is crazy.