"Mas despacio, por favor!"
Incidentally, speed is precisely why I gave up Portuguese. The gal has a horrid case of word vomit. :(
Ah, that's not a reason to give up a language. I just came back from Spain and in the two weeks there I learned that they speak even faster than the Duo lady... But if you ask nicely (or just look really really lost and confused) the people will repeat it a 1000 times and use their hand and feet to make you unserstand.