There is something SERIOUSLY wrong with your bike. I drive a 250 cc scoot (Piaggio MP3), that accelerates fine with passenger to 65. With a top speed of 75, however, acceleration to that speed is S L O W. All my Harley ridin’ friends love it and often want to trade for a while during a ride.
Nothing wrong. Just had it at the shop for an oil change and adjustment. 624cc is just way underpowered for a four passenger vehicle.