I don't think their love was rushed at all. In my opinion, i'ts clear that Simba and Nala like each other as cubs. They just don't understand the concept of love yet. After Simba disappears and we see Scar telling the lionesses what happened, Cubby Nala is balling her eyes out. I mean, Simba was her best friend but wouldn't that be a clue about her deeper feelings for him? Anyways, after finding out that Simba's alive, Nala even confesses how much she's missed him and how much he means to her, so again, she obviously knew that there was more than friendship there to begin with. If you had a best friend who you liked when you were younger and thought they were dead, only to be reunited with them quite a while later, I think that's more than enough to spark a romantic love and in my eyes, it's justified.
