
Neville or Hermione, for example, would not fly really well even on a Firebolt, so I guess it depends on the rider as well. On the other hand, the performance of a broom cannot depend exclusively on the magic imprinted on it by the maker.Under the same logic of "brooms driven by the same magic should have the same speed" we could say that all wizards should be able to perform the same spells at the same stage of their education (or worse: same effects for the same spells, all the time).Some brooms are faster than others more or less for the same reason some wizards achieve better results than others when casting the same spells: they "have more" magic. Nimbus has been at the top of the field since the introduction of the Nimbus 1000. QTTA - pages 50-52 - The Development of the Racing Broom

Reaching speeds of up to a hundred miles per hour, capable of turning 360 degrees at a fixed point in mid-air, the Nimbus combined the reliability of the Oakshaft 79 with the easy handling of the best Cleansweeps. Nothing like the Nimbus 1000 had ever been seen before. Regarding the Nimbus brooms, according to QTTA, the Nimbus Racing Broom Company: Thus, you will see a difference between the different brands of brooms. Some broom-makers will have stronger or more specialized magic than others, I'm guessing. Brooms are able to fly due to charms placed on them the skill and power of the witch or wizard imbuing the broomstick with its flying powers is going to vary. Just as wandmakers learn their craft through apprenticeship, perhaps broom-makers are the same and learn their craft over time. Well, as the passage above notes, some witches or wizards were more skilled at making flying broomsticks as others. Why were there speed differences when broomsticks were driven by magic? QTTA - pages 2-3 - Evolution of the Flying Broomstick Once broomsticks became more comfortable, they were flown for pleasure rather than merely used as a mean of getting from point A to point B. By the twelfth century, however, wizards had learned to barter services, so that a skilled maker of brooms could exchange them for the potions his neighbour might make better than himself.

As Wizarding families in those days made their own brooms there was enormous variation in the speed, comfort, and handling of the transport available to them. The charms placed on a medeival are similarly basic: It will only move forwards at one speed it will go up, down, and stop.
