Ask HN: How to motivate people when you think they could be doing better?

I am a startup founder with engineering background. I found myself moving more and more to the managerial position as the company grew. One of the biggest worries today for me is how do I make people happy and motivate them to do their best. I have the opportunity to work with extremely talented people but sometimes progress is just not as good as I expect it to be. My personal benchmark is to think how long I would take to do the task myself, double it and then compare for what has been delivered. I know that saying "do more" or "do it faster" do not work with highly intelligent people.

Hence my question: how to motivate people when you think they could be doing better without saying do "better / faster"?