Get people involved and engaged. A great way to do this is leading questions, like "What do you do?" or "Do you have any questions?" after describing a situation. If players get stuck, offer them some suggestions for things they could do or review the things they can see, hear and found out recently.
For players, try to keep track of what is going on and make an effort to do things that move the story forward. Even a bad idea is better then trying to think of a perfect idea or endlessly debating your options.
One thing that can massively slow a game down is if players have to declare searching for traps and general, routine precautions. If every hallway, door and area they come to has to be swept for mines with a visual inspection, followed by a chem sniffer, the game will move like grass growing.