I'm not sure I agree with this. Kane isn't fast at all, and he's one of the most successful strikers in the Premier League. He is strong, of course, and has excellent hold-up play, but for a 20 year old striker, so does Højlund. He is absolutely fast enough for PL.
He might not be strong enough yet, but he has very good work rate, got great intensity in his play and according to staff, he has a warrior mentality and attitude.
Put him in the gym and he'll be strong as an ox in no time. Not that he really needs to be, as his hold-up play is pretty good (think Martial - not particularly strong, but very difficult to get the ball off of him when he has his back to goal) and very quick.