Party Leadership

How do you (as DMs) deal with a refusal or inability of anyone in the party to take on a leadership role? Their decision making process is painfully slow and frequently derails momentum of a session. 

I have found giving them limited choices, frequently and often, helps. I've also found it helps sometimes to ask each member individually what they think and moving on if you don't a response and then saying ~ like so it seems the consensus is X, wait a moment ... no objections, I assume it's X and start narrating from there (if they wanted to do y instead they should have said so). 

However, it's never been a major problem for me. More an irregular occurrence & there's always been a few players who will sort of take the lead, if not all the time or always the same people. If it's particularly bad you could either talk to an individual in the group who you feel might have the qualities to lead or talk to the group as a whole and ask them their own opinions on the issue and say how you feel and come to a solution together. 

Hope this helps.