The startup environment comes with some downsides; it is not for everybody, especially for those happy in a comfort zone. Dealing with work colleagues who were not adapting fast enough to your and the company's rhythm was a little bit difficult.
I agree that sometimes the speed of the company can be overwhelming; it requires you to be fully awake and ready to adapt. This is exhausting, but on the other side, it is what forces you to learn fast and keep improving your CV. This downside makes me a better professional but is not always easy.