Fast Lenses
Many people seem to think that the maximum aperture of a lens tells you everything about the speed of a lens. This is an oversimplification. Maximum aperture does matter, but one also must consider the light-gathering power and the transmissivity of the lens.
A camera lens is really no different optically from a pair of binoculars. For better performance in low light at the same focal length, binoculars need to have larger objective lenses. This is true also for camera lens: the light gathering difference in f-stops for two lenses will be roughly the ratio of the squares of their objective element diameters. For a rough estimate here, use their filter sizes.
Transmissivity is harder to determine, but basically, each five additional coated lens elements in an optical path will subtract about half an f-stop from overall lens performance. Glass is in fact imperfectly transparent; for otherwise we we would never be able to see a lens.
So, if you want to know whether one lens is faster than another at a given focal length, then factor in their light gathering and their transmissivity as well as their maximum aperture. Faster lenses will have bigger objective elements in front and fewer elements over the entire optical path.