Comfort bikes usually have a very ineffectual riding position, not just one that sits you up straight. If you look at a very comfort-oriented but practical bike like those used by Dutch people, you'll see some key differences.
Comfort bikes almost always have a seat tube angle that's too steep to work well with the upright seating position. They customarily come with a lame pogo-stick fork and seatpost, both of which make the ride worse and less controllable while pretending to make it better. They almost always have jointed adjustable stems, which are weak and become loose and wobbly over time. They commonly pair V-brakes with short-pull brake levers (the wrong kind), perhaps because their manufacturers think comfort bike buyers are weaklings.
A Dutch style bike will have slack frame angles and generous fork offset to go with its upright riding position. It will use the largest available wheel diameter and generously large tires, rather than hokey suspension components, for good ride quality. And it will come with relevant features like skirt guards, fenders, chain case, drum brakes, generator lighting, and provisions for luggage, all of which make the ride more comfortable and convenient in various ways.
Neither one of these types of bike is intended for fast riding. But one of them is a much better choice for going fast. I think it comes down to Dutch bikes being made for people who actually ride for transportation every day, while comfort bikes are made for selling to people to think it would be a good idea to get more exercise, as long as it doesn't take much effort.