The most dangerous thing you can be on the road, after drunk / high of course, is impatient and frustrated.
An angry driver is a dangerous driver. Calm down, hang back, and overtake when / if safe and legal to do so.
Someone else doing 50mph in a 60mph zone, whilst not ideal for you, is not a crime, and a vehicles top speed will depend on the driver, the vehicle, and the road conditions. Even if you see a modern car with a perfectly healthy looking youngish adult driver, you don't know if they have an injured or unwell child or animal in the back (for example), or fragile cargo, and are taking things more gently.
So focus on your own driving standards, and not those of others. If you think someone is going too slowly then hang back, and wait for a safe and legal place to overtake. If there isn't one, then just hang back.