Relationship Compression

This trope happens when an adaptation (usually a film or musical based on a book), due to medium constraints, has to compress the time a romance has to develop. This can lead to accusations of Fourth Date Marriage, Love At First Sight, or even Strangled by the Red String that are not an issue in the original. Adaptation Displacement will exacerbate such complaints, even extending to Die for Our Ship.

Not to be confused with Promoted to Love Interest, as the romance was present in the original, but its speed is just altered.

Examples of Relationship Compression include:

Comic Books

This happens in Scott Pilgrim vs. the World (comic book to film) with Scott and Ramona's relationship. Originally, their relationship built up over a year or so before the comic ended. In the movie, the same development had to be cut down to a few weeks.

Literature

Raoul and Christine in The Phantom of the Opera (book to musical): The two may be Victorious Childhood Friends in both versions, but the two spend hardly any time together on-stage as adults, compared to the longer, more complex (read: painful) progress of their relationship in the novel.
Marius and Cosette in Les Misérables (book to musical): Marius seeming to fall in Love At First Sight with Cosette in the musical has strongly contributed to making Marius/Eponine the Fan-Preferred Couple.
- Of course, in the book, it's not much better. Marius and Cosette meet each other's eye a few times, but don't say a word to each other; yet when Valjean moves to another part of town and stops frequenting the park where Cosette and Marius would catch sight of each other, Marius and Cosette both fell into depression, and Marius resorted to stalking and hiring people who knew the area to catch glimpses of Cosette again. At one point, he leaves a notebook filled with his musings on love for Cosette to read; they're clearly talking about her, and are generally expressing the sentiment that life isn't worth living without love -- and he still hasn't spoken to her yet, and doesn't even know her name. Not long after their first conversation together (perhaps days), the two are saying they'll die if they don't get to see each other again.
Eowyn and Faramir in The Lord of the Rings (book to film): In the book, they get more page time than even Aragorn and Arwen, but in the movie, they're a one-scene Hooked Up Afterwards. The extended version improves things a bit, but not much.
Lupin and Tonks in Harry Potter (book to film): In the book, their relationship is an (admittedly mostly off-screen) subplot, involving much angst about the possibility of Lupin passing on his werewolf gene. In the films, there's just a throwaway mention that they are now married and later a throwaway mention of Lupin having a son.
The 1980 film version of Flash Gordon has Dale and Flash falling for each other at an alarming rate. Lampshaded by SF Debris who questioned the strength of a relationship built up over one long afternoon.

Theatre

One of the most famous examples is Shakespeare's Romeo and Juliet (book to play), where the romance between the titular couple was compressed from happening over several months (the time given in earlier versions of the story) to a few days.
Sweeney Todd (play to film). In the stage version, the romantic subplot between Anthony and Johanna is significantly trimmed down in the Tim Burton film, which cuts several of their songs. With the film version being left with Anthony simply seeing her face through a window once and then instantly devoting himself to her.