There’s no ‘fastest way’ to mow a lawn, but there are several ways that will cause you to cut the lawn faster than others - although, this depends on the size, style, and design of your lawn. The most common method is to cut in straight-lined rows. Cutting your lawn in rows ensures that you get every inch of grass you need, missing no patches and reducing the number of times you need to turn (or reverse). There are several zero-turn mowers on this list that can help you cut efficiently.
However, this doesn’t always work for some people due to the shape of their lawn. Another option is to cut in concentric circles. Aside from being more fun (unless you’re prone to bouts of dizziness), it also reduces the number of turns you need to take and is in fact simpler than rows as you are always turning in the same direction. If there are obstacles that interfere with straight and simple mowing, do them beforehand so you can get on with the longer, more arduous mowing tasks and feel complete once you finish everything.