Speed is only an advantage if what you're building has value. If you're building a crappy product that nobody wants, building it faster isn't going to help you make money.

Also, the generalization that "startups are fast, big companies are slow" is a bit misleading. In many cases, big companies are building vastly bigger things than some startup trying to put out their first MVP. For example, if a big company like Facebook wants to roll out a new feature, it has to be built at a scale where it can be used by a billion people - which takes a lot of engineering. If a startup wanted to build a product to directly compete with Facebook, they'd have to deal with all the problems that Facebook deals with, and probably wouldn't have a speed advantage.