The F Ratio is the ratio between the focal length $ f $ and the diameter $ d $ of the lens. Strangely, $ f/6 $ means that $ f/d = 6 $
A small F Ratio corresponds to fast optics because $ d > f $ which means that more light will pass through the apperture, reducing the time it takes to make an image.
A large F Ratio corresponds to slow optics because $ f > d $ which means that the same image will be spread across greater area. To get the same amount of photons per area, we need more exposure time.