What was so different between the two acts?
I don’t share GP’s opinion, I quite enjoyed both acts. I do agree that the pace of the two is quite different - the first being a bit more sci-fi thriller pace and the second is a bit more contemplative.
I agree, though. The first two acts were great, and the third fell flatter than the earlier two. But I enjoyed it nonetheless. It's nice to imagine what would happen, and it's pretty hard scifi throughout, albeit greatly accelerated.
Beautiful worldbuilding, but very much not like the rest of the book.
“[The characters] spend most of the third part literally doing nothing. They are assembled into a team of seven by a nebulous authority for some secret purpose; neither they nor we are told what it is. They go from place to place to investigate something or other, making several pointless stops and excursions and wondering, as I did, what was going on and when something was actually going to happen.”
But so much of my time was spent reading about those people going from A to B to C on their way to D, and for what?
There was a similar episode in _Anathem_, which I generally liked. But for some reason the characters had to go from A to B and the best way to get there was to go over the North Pole, and the book stopped for fifty pages until they got over the Pole and everything could continue.
In a different world some editor would have persuaded Stephenson to cut the whole thing out, and I used to hope he would one day meet that editor and we would live in that world, but by now I've given up.
I would have liked to have seen something like that in Anathem: two sentences along the lines of “Then we had to get to Siberia by driving an APC over the North Pole, but the less said about that the better” and then the book moves on.
Although in fairness, the person you're replying to is probably doing the same.
I liked it, but I can totally see the criticism.
After all of that resolves, it's pretty hard to emotionally connect with the rest of the story, since you are exhausted from all that came before. The final act is worthwhile and interesting, it's just a pretty big change in direction.
The second part is a war between humans for scarce resources in space.
The third part is about humans, who are largely doing fine, finding out stuff about the past and themselves.
There are some deadlines & urgency in the last part, but I think it's a tall task to try and create urgency when the previous sections were about the entire human races being extinguished.