He is definitely not slow, but using top speed as a measure of football applicable relevant pace is just plain wrong. It means practically nothing. There are very few moments in the modern game where any player runs 50-70 meters in a straight line which is what it takes to even reach your top speed.
In 2018-19 the player reaching the highest top speed in the PL was Fosu-Mensah for christ sake, not who you would classify as a high-pace merchant:
https://www.sportskeeda.com/football/premier-league-fastest-players-from-the-2018-19-season-revealed
Haaland has a very similar type of pace that Lukaku had when he burst on the scene and hadnt put on muscle yet. Haaland has physically matured already and if he is going to keep his pace he needs to not bulk up much more in the next couple of years.
To say that Haaland beats players like Salah or James, or even Rashford for pace over a 15-20 meter sprint, which is football-relevant pace, is ridicilous.