I think acceleration is a better metric than top-speed for attacking players. Of course, if a player excels in both, he becomes a Kylian Mbappe.
The top-speeds of Havertz (now) and Hazard (2016/17) are similar - 35 km/h, which isn't much because both Rashford and Greenwood have faster top-speeds (36 and 37 km/h). But prime Hazard had that sudden burst of speed from an absolute standstill position (acceleration), which enabled him to skip past every defender in the league, much better than Rashford ever has ever done. That is what Messi excels in. He is not the quickest on the field but has better acceleration than most players. It more than compensates for the lack of top-speed. Havertz strikes me as a player who isn't the quickest over a long distance but has that enormous burst of pace in tight spaces, which is absolutely crucial - this isn't a 100-metre sprint, but a game of football.