After you get Nancy's spy binoculars, you want to go back to the Ca'. Fango shows up at the same time Nancy does. When you go to the roof to spy on him, the game forces you into a conversation with Margherita.
The first option is "I've already been up for a while", and it's 3.6 seconds longer than the other option, because Margherita will respond with a bit about her morning routine, before talking about tanning. If you pick the second option, she talks about tanning directly.
Margherita will ask if you like the room, then she will ask if you mind having a roommate. If you complain about the room, it takes 11 seconds longer to get to the roommate question. If you say you like having a roommate, Margherita says you're just being polite, which takes about six seconds longer than saying you don't mind. At this point, Margherita warns you about locking the doors, and you can end the conversation.
Margherita asks you what you're doing, when you spy on Fango. You have three options, which all result in a different conversation later on. Uh oh. To figure out which one is fastest, I'll have to replay that part of the gave three times!