The speed of a lens is actually determined by the size of the aperture. When describing lens speed on any photographic lens, it is expressed as the maximum aperture diameter - or the minimum f-number.
A Little More Detail:
So why do we use the term lens speed when it seems to be all about aperture size? A fast lens with a larger maximum aperture transmits more light to the focal plane and reaches the same exposure with a faster shutter speed compared with a slow lens with a smaller aperture. The faster the shutter speed the less blur and noise you’ll achieve in the photograph, which is one of the perks of a “fast” lens.
For example, lenses under F2.8 are considered “fast” and are usually more expensive as quality design and construction are needed to achieve larger aperture diameters, especially on zoom lenses with longer focal lengths.