Demorou comes from the verb "demorar," meaning "to take a long time" or "to delay." While it is a common verb, its use here is particularly poignant because it turns a simple matter of timing into an accusation of lost opportunity.
The lyric "Até te esperei, mas você demorou" ("I even waited for you, but you took too long") uses the word to show that waiting itself had a limit, and that limit was finally crossed.
Ever missed someone who doesn't exist anymore? That's the feeling at the heart of Wesley Safadão's "Amor, Ex Amor." The singer isn't missing his ex-partner as they are now, but rather the person they used to be. He points out how much they've changed, saying, "My longing isn't for you / It's for someone you once were."
He suggests that while his ex might be popular, they are ultimately lonely, asking, "What's the use of having a heart for the crowd and living alone?" The singer waited, but his ex took too long to realize what they had. Now, it's too late. Someone else has stepped in to care for him in a way his ex never did. He calls them his "love, ex-love," declaring that their past relationship couldn't have been true love if it ended, and that his ex's actions were like asking "to lose me forever."