He isn't slow, just not fast enough to make up for his average dribbling. Best wingers can use both speed or dribbling to beat their guy, Ferran is meh at both.
And due to his bad finishing he can't even compensate being a mediocre winger with more goals.
I kept defending him because you have to do something right to get into good scoring positions in the first place but it's just not enough.
Not like he's the only one for Spain though, Olmo - Asensio - Ferran is just an all around pathetic front three and it's not like Morata would make that any better
Even with an Ansu from a different timeline without any injuries they would probably still struggle to score.