You’re definitely not alone in that feeling. The film effectively shifts from a Gothic ghost story to a procedural courtroom drama, and for many, that transition breaks the spell.
The first half is pure atmosphere—it's Hitchcockian suspense at its best, where the "ghost" of Rebecca is everywhere in the shadows and the cinematography by George Barnes makes the house feel like a living monster. Once the mystery is "solved" and the legal investigation begins, the movie loses that haunting, claustrophobic quality.
Here is why the second half can feel like a bit of a letdown:
The Loss of Mystery: Once Maxim explains everything, the enigma of Rebecca is gone. She stops being a terrifying, supernatural force and becomes a human villain with a medical record.
A Shift in Power: The narrator goes from being a vulnerable, relatable protagonist to a stoic accomplice. While it's "growth," it removes the tension of her being the "underdog" in her own home.
The Censorship Hit: As you noted earlier, the movie's need to make Maxim "innocent" drains the stakes. In the book, the tension in the second half comes from the fear of them getting caught. In the movie, since it was an accident, the legal drama feels a bit more "manufactured."
The "Manderley" Effect: Much of the second half takes place in offices and cars, away from the set design of Manderley, which was arguably the most interesting character in the film.
Despite that dip, the visual of the burning "R" on the pillowcase at the very end usually brings back enough of that Gothic magic to leave a lasting impression

Well said. It was a great combination.
Mag, Darling, you're being a bore.

I thought that the movie was great from beginning to end. It was a combination of romance, mystery, thriller, etc.
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JimHutton (1934-79) and ElleryQueen

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I realized when I read my earlier reply to you that it may have sounded like I was favoring plot over characterization. But that is not the case. I think good (really good) motion pictures have compelling characters and compelling plots. In the first half, almost the first two-thirds of REBECCA, we get glimpses of all these compelling people but nothing much happensthey meet, get married (off screen) and then she learns the backstory of his first wife.
But it's not enough to know these people and their circumstances. Something needs to happen. Maybe if we had a scene where Olivier has a nightmare about Rebecca, which leads him to think it was mistake coming back to Manderley. Or maybe if Mrs. Van Hopper turned up and warned the new Mrs. de Winter what she's learned about the death of Rebecca, then we could have seen our young bride have a reason to think she's gotten in over her head. More needs to be done with her thinking her husband is guilty.
As I said previously, I don't feel it kicks into high gear until the boat is found. I will admit that some of the earlier scenes are quite good, like her seeing Rebecca's bedroom for the first time; but many other scenes are dreadfully boring the part where she goes into the room where the fireplace isn't turned on till the afternoon was completely unnecessary and even her scene after that in the morning room looking at the stationary on Rebecca's desk was drawn out and could have been done in ten seconds with a quick dissolve, not two or three minutes of stretched out nonsense (we get it, she's learning about the morning rituals of the former wife, okay, but move the story along already).
Then, there's the introduction of Nigel Bruce's character and his wife which also goes on too long, not to mention the introduction of George Sanders' character which hints at the last act; the endless scenes with the dog that practically disappears from the later action; the costume party (which is necessary but sort of plays like a spoof of a southern belle with Fontaine giving a Scarlett O'Hara imitation probably a costume leftover from Selznick's GONE WITH THE WIND); the supposedly happy couple watching the honeymoon video (twice!); and so on and so forth. It just takes too long for this movie to get going. But once it does, around the 80 minute mark, it takes off. I can see modern audiences finding it much too slow in the beginning and giving up on it, when the final hour is truly sensational and classic filmmaking.

Thanks for your reply.
I know the action starts about 80 mins in the film , but I don't know, something about the story following the discovery of the sail boat and Maxim's confession to Mrs. de Winter lost a bit of my interest. The answers seemed all too simple for the delusive Rebecca I had been led to envision.
I guess I just preferred the more moody , character-driven 1st half (which reminded me a bit of Sunset Blvd., one of my favorite films) . And Joan Fontaine was so beautiful in this.
Actors are like cows. You have to lead them through a fence.

I feel the opposite. I think the first half hour with Mrs. Van Hopper should have been condensed into ten or fifteen minutes where we get to know Max, but most of it is compressed into a long montage. As it is, some of those scenes, while charming, were overly long and the action really doesn't start until they are on their way to Manderley.
At Manderley, I felt that we were getting to know the house and the background of Rebecca's life there in bits and pieces, but it was very stretched out. I looked at the clock, and the discovery of the boat and the other body does not happen until the 80 minute mark. Most motion pictures are winding down at 80 or 90 minutes but in this case, the film is just reaching its turning point. The last half-hour, when we have Favell's blackmail scheme and where we find out whether or not Rebecca was pregnant is truly the most compelling, plot-driven portion of the movie. Then the ending with Danvers going mad is the most dramatic, climactic part.
This film could easily have been trimmed by thirty or forty minutes and it would have been just as good. What we have in this form, is producer David Selznick forcing Hitchcock to aid him in indulging in continuous atmosphere and a padded story that is meant to be big and shout masterpiece. It is definitely a masterpiece, but it could have been streamlined in parts and been a lot more compact without compromising any of its value.