How fast do Scooters go?
Small 50cc scooters ("Mopeds") are good for about 30 mph, maybe 35 mph in some states without speed restrictions. They will just about keep up with urban traffic, but you have to ride them WOT (Wide Open Throttle) do do that. This can be good for novices and young riders because it's harder to kill yourself at 30mph than 60mph, but the inability to keep up with faster traffic can sometimes be a hazard as cars try to "squeeze" past you when there really isn't room. I've ridden 50cc scooters and their lack of speed can be a problem at times when the traffic is moving at 30 mph and up.
150cc Chinese scooter. It might look fast, but it's not really very happy above 50 mph.
Mid sized 150cc scooters are good for 50-55mph, though with a long enough run at it they may get up to 60+mph, especially if there's a tail wind or slight downhill grade. My chinese 150cc scooter has been up to 55mph (as indicated by my pocket GPS, the speedometer showed 60+). I don't think it was topped out and with a little more time it may have made it to 60mph! At 55mph it was OK, but I didn't feel particularly stable and I don't think I'd want to ride at that speed for any length of time.
45-50mph is enough to keep up with most urban and suburban traffic, though it's really not enough to flow with the traffic on a freeway, where speeds of 65-75mph are more normal. I'd certainly recommend a 150cc scooter over a 50cc scooter for commuting unless you live in Manhattan. My 150cc scooter cruises quite comfortably at 45-50mph. I'd be reluctant to ride it with WOT (wide open throttle) for an extended period of time, as running at top speed puts additional strain on the engine and 150cc scooters really aren't intended for high speeds. If you want a high speed machine, you would probably be better off with a motorcycle. For around $3500 you can get a Kawasaki Ninja 250, which is capable of reaching about 100mph and will cruise comfortably at highway speeds.
250cc scooters are just about OK for freeway use and should be able to cruise at 60mph and get up to speeds of 70mph or more if pushed. I wouldn't recommend riding anything less than a 250cc scooter on the freeway, and a 400cc scooter would be even better.
Not a scooter at all, the Kawasaki 650 Ninja is cheaper, lighter and faster than a 650cc scooter
Larger scooters go faster of course. The $9000 Bergman 650 will cruise at 80 mph and can go as fast as 110 mph. However if you really want to go that fast, you'll get better handling out of a motorcycle and it will probably be cheaper. The Kawasaki Ninja 650R has a price of $6500 and a top speed of 138 mph. Compared to the Ninja 650, the Suzuki Bergman scooter is a lounge chair on wheels!