Finally had a chance to see this.
Same here: I liked the first part. I think it’s a mostly a script problem with the third act (note to screenwriters: if you devote the better part of an act to “… and then they were in a happy relationship for a bit,” then you need a rewrite. A long period of contentment might be soothing for the characters, but it’s boring to watch for the audience. It’s the opposite of drama. Condense that down to one scene or a montage.)
It’s also partly that Swinton and Miller, as enormously talented as they are, just aren’t the right fit for what is ultimately a romance. And it turns out Miller isn’t the right fit for a movie where a big chunk is a bunch of conventional dialogue in a room. The scenes showing the stories of far-off places soar, with great composition and a swooping camera. Then we cut back to the hotel room and it looks like a TV show. And not an HBO one.
But a lot of it it ultimately is due to it being based on a story by A.S. Byatt. The sort of 90s swooning-magic-realism-by-way-of-the-university-library academic lit fic the movie is based on works on the page (at least when the reader is of a likewise academic bent) but it’s much, much harder to pull off on the screen.