Translation:She is not fast.
はやくない can actually have two meanings. Depending on the Kanji used, the un-conjugated word はやい can be 早い or 速い. If Duo used the Kanji, this would be easier to differentiate. 早い means early, while 速い means fast. So in this case, if "The girl isn't fast" didn't work, it would be "The girl isn't early." But I would also say that the context would imply the phrase means "She is not early".
That's the major problem with Duo. They need to start using the kanji more, to keep this kind of confusion down. It's hard to glean the correct meaning with so many homophones.
I'd assume that with the context, the word かのじょ means 'she' and not 'my girlfriend' because there is no possessive to be had. If it was 私のかのじょ I would think that would be 'My girlfriend', but that's just my speculation. It's all dependent on context from the rest of the conversation to be quite honest.