Grammatically it can mean either of the two, (1). "I think/heard that it is fast, can you confirm that?" and (2). "Is it not-fast? (Do you think that it's slow?)" But in 95% occasions , 速くないですか means the meaning (1), "It is so fast! (isn't it)". (I'll write about the reason, later)

Even if you should see a foreign language just from the viewpoint of a native speaker of another, it would be enough for you to learn it as a second language. You can never speak a foreign language as native speakers of it do. You don't need to at all. So, what is the most important for those who answer questions of non-native speakers here is to explain something not by using only technically precise words and phrases that they don't know well but by using simple words and phrases they already know well or can easily understand.

You would not be here at this website if you really wanted to become a pundit in linguistics or phonetics. I guess you would just like to be quite good at Japanese as your second language. If so, you would not need to spend much of your precious time to learn about the precise differences between Japanese and your mother tongue. It would be too much except for some language "otaku", whom you can often see just showing off his trivial knowledge dogmatically.

Ah cool, that's really useful to know! Thanks

Just to let you know a bit more, the pronunciation differs depending on what it means. If it means "Isn't it fast?", "速く" is most stressed in the sentence. If it means "Do you think it isn't fast?", however, "ない" is as much stressed as "速く" is.