When you speak to an Xfinity rep on the phone they will always answer any question about internet download speed with the phrase "up to". "Well, you will get UP TO 25mbps, with this plan".
In fact, the chances of you getting anywhere close to that, when you need it the most, is slim at best.
Sure, you can test at 6AM and you will see your speed at, say 20 mbps or even 24 mbps. Test it again at dinner time and you may be in for a shock.
The reason is simple. The more people using their connection, the slower your speed (and theirs for that mattter) will be. This is a fact of cable life. So, when you need it the most, like in the evening when trying to stream a show, it will likely not be there for you.
BTW: The best test site out there is speedof.me. Try it for yourself. It saves your results for you so you can compare it every time you test.