Bikes are generally fast, and how fast they are good depends on the type of ride you have.
1.If you only ride occasionally, 18 speeds are sufficient, 18 speeds are 3*6, which means that there are 3 options (pedal forward 3 times) and there can be 6 combinations (pedal forward, pedal back, pedal left, pedal right, do not pedal, pedal forward and pedal backward at the same time).
2.If you want to ride longer distances and want to have a better workout, 21 speed is more suitable, 21 speed is 3*7, 4 options can have 7 combinations.
3.If you wish to go on long rides, it is recommended to use the 24-speed, as the 24-speed is 3*8 and there are 8 combinations of 5 options.
4.If you want to do very intense riding, such as racing or mountain biking, it is recommended to use the 27 speeds, because the 27 speeds are 3 x 9 and there are 9 combinations of 6 options.
In short, how fast you can do it depends on your riding needs and budget. If you're only riding occasionally, 18 speed is sufficient;If you're looking for better results, the 21-speed is suitableIf you're looking for longer rides, a 24-speed is recommendedIf you want to do very intense rides, such as racing or mountain biking, the 27-speed is recommended.