“…they stumble that run fast.”
I think Friar Lawrence is trying to get Romeo to understand that he needs to take a step back and look at his situation before marrying Juliet. Romeo was also deeply in love with Rosaline until he met Juliet. I think it also refers to how fast he changed between the looks of two women. Romeo is clearly rushing things seeing as he only met Juliet a day before wanting to marry her, and I think this quote says that no matter how in love they think they are if they rush into things they will be faced with a problem.