I don't think that speed is the first reason. I think startups using the dynamically typed languages are using them because the first developers are familiar with them. A team that are familiar with Java can code at same speed as Python developers (Java can be longer but it just typing, coding is not plain typing.)
Of course there are advantages and disadvantages, but if you are starting from scratch and if you don't know if it will scale or not, it doesn't matter that much. Just go on with the one that you know. That's what Quora founders did.