Also, right now I accept that she's always late because I know she runs over with just about all clients. If she were a smoker, I'd probably feel resentful and think she's late because she was having a cigarette.
I might also be afraid of them resenting me for getting between them and their cigarette if I run over, ask them to call me etc.

And would they be paying their full attention to me, or thinking about their next cigarette? Maybe even trying to finish the session two minutes early?

I mean, my pdoc and I have appointments of an hour. We often speak for about 45 minutes (and sometimes for up to 75, if there's a lot to talk about and he has time). If there's nothing left to talk about, we finish up. I leave as satisfied then as when we take the full 60 minutes, or when we go over time. If we finish early and he went for a smoke or I considered he might, I might wonder if finishing up early was on purpose. Did he rush the session in the hopes that ..? Did he maybe not pay attention to something I said-did-showed because ..?

I'm not sure if I would actually think that, but I might. If I feel someone considers me a bother, I'll try not to be bothersome - often at my own expense.