There could be many reasons how they're taking off quicker than you.
Sure they may have modified their engine or done something with the air filter but also they might just be quicker than you on the gas pedal. But sometimes when I try to be quick on the pedals I don't give enough gas then I stall and humiliate myself... lol. Easy ways to increase acceleration are to remove any junk in your car.. like if you've got any bags or whatever taking up room and weight in your car.
If you've got some money to spend you could buy a custom performance exhaust, the more you pay the better it will probably be on your acceleration. They can increase your BHP by around 5 to 10, sometimes more. New air filter, plugs, wires, fuel filter may enhance the exhaust, but will not add much standing alone.