I stayed away from Arduino for a long time, I was under the impression that it was running on an interpreter. But its compiled and like H500 said, its just some of the libraries that are slow. Whats so nice is the dirt cheap dev boards and HUGE user community. If Arduino is still too slow after some work you can just port the code over to a teensy3 or one of the other ARM-duino's out there. Cost-wise they are the same.