Its a 125cc and could barely do 40... Dafuq? My scoot is a 50cc and it cruises at 42 all day, I've had it clocked at 51mph by one of those digital speed limit signs near schools. All I've done is put an exhaust on and didn't even rejet the carb. Pretty sure the dude you bought that thing from was just looking to make a quick buck.
If you want a bike that can keep up with traffic but is still small enough to learn on, I'd go with either an old body style (88-07) Ninja 250 or a Buell Blast (small 500cc). Or if you want to stay with the cruiser style, a Honda Rebel (250cc). All three of which can be had for cheap as hell.