Speed is inspiring
Patrick Collison keeps a great list of "ambitious things" accomplished in impressive feats of time.
Speed minimizes exposure to volatility
Lots of things can go wrong, particularly in startups. These may be controllable things, or completely external factors. While time may be fungible, timing is not.
Moving fast builds momentum
Effective decision makers and leaders typically have their own solid framework for decision-making, and are decisive. This means that they have a set of heuristics - developed, inherited or otherwise adopted - which enable them to act fast and with conviction. And they value execution in line with their framework. These frameworks may be formally expressed in a handbook; in dribs and drabs through strategy letters, blog posts, and in interviews; or informally held (perhaps even unconscious and unrealised). But harboring strong beliefs and operating principles that translate into a particular way of work (not necessarily the same one across different successful organizations) appears to be a commonality amongst top leaders.
Being able to make decisions fast frees up time, and enables more decisions and actions to be taken overall.
Making decisions fast unblocks dependents faster, and the benefits to speed compound.