You're absolutely right. From the moment they arrive in 1955 in part 2 it's so tightly paced. And having Marty interact in scenes from the first movie is fucking genius.
I was thinking of this shot in particular.
It's amazing how well constructed it is. The multiple stages of foreshadowing. First you have the sky storming in the background. And what's incredible is how they don't draw your attention to the storm. It's just THERE. Flashing in the background, looming in every shot. The cinematography and general visual design of the trilogy is amazing stuff. Back to the Future as a series created sequences that built to a climax in a truly effective way. The music, the acting, the visual design, and in this case the tree being struck by lightning as a pivotal moment like a dream turning to a nightmare. The tree is struck, the audience gasps. Doc remarks that he almost bought the farm there. Then Marty strides forward, declaring, "You don't want to get hit by lightn--"
The Delorean is struck by lightning. If executed wrongly this would have been extremely dumb scene. Dragging out the inevitable. Or coming from nowhere with no buildup. But the pacing, the editing, and the portrayal of the storm all works together to create an event you saw coming -- there's so much foreshadowing -- but which is like being hit by lightning.
The train sequence in Back to the Future III is similarly brilliant.
The pacing is exquisite. The colour coded logs exploding provide stakes, the slowly rising speed call-outs, and a sense of the end coming just like the end of the track is looming. The character's fates change moment to moment. This is the kind of pacing and editing that makes something like the train sequence from Wallace and Gromit: The Wrong Trousers be held in such high regard. There have been films that try to recapture the magic of this particular train sequence. The Lone Ranger and Zorro 2 created shameless homage/ripoffs of it. The peril. The stakes. The moment our character comes to a realization about who they are and who they want to be. But I don't think any film has topped BTTF3's version. It's just incredible.
Even the way the film effortlessly feeds the audience exposition about the presto logs and the windmill point of no return and stakes is graceful. The hoverboard saves the day. Instead of feeding the audience foreshadowing about the hoverboard, we see a single glimpse of the hoverboard falling into place near Marty's foot, with the scene's incredible kinetic energy remaining unabated, like a runaway train, as it were.