Just my opinion and from my experience of being a juror in a murder trial.
The one requirement of the jury is to make a unanimous decision. If they can't do that then they've failed. So, it is important that very early on they all know where each other is at in terms of who thinks what. Some may still be undecided. Very quickly, if not straight away they would realise its important to know where each other is at because otherwise they are just wasting time. They've all been sitting there for 10 weeks or so, so would be pretty keen to get to the business.
To answer your question about if all jurors agreed right from the start, my feeling is that they would definitely still be going through the requirements of the charge (if guilty) or the judge's directions but certainly wouldn't need to take too long going through everything. They don't need to talk themselves out of the decision but definitely need to do some checking to make sure they've clearly made the right decision.
It there are disagreements, then they'll be going over the points of contention, and that could take a while!
The fewer jurors that are against the majority, the more the pressure will build on those people or that person. Though, from my experience, considering what they've all been through, they'll all be very supportive of each other, even if there is stubbornness within the group. They've been through the trenches together and need to get out in tact.
It is an enormous mental burden for jurors and I can imagine this case will have taken its toll an a few of them already.
All my opinion