I disagree; it depends on whether the player values maneuverability.
Some people, for example, like that they can throw almost their entire army into the attack on the first turn and win. This makes the game more dynamic, but less tactical.
However, simply reducing the top speeds is not enough, because after that, the problem of uniform speeds arises, as in Heroes 6 or Heroes 7.
Therefore, a small reduction in infantry speeds, around 20-30%, would be appropriate (but across the entire range, including actively using a speed of 3). With the exception of flying units, whose mobility should be distinctly high (as in previous installments), but with reduced damage or survivability. In this case, maximum speed diversity can be achieved, flying units can have a distinct class role, and the problem of "half the army attacking on the first turn" can be solved.
And, of course, if you add even more conditions, you can balance it back to how it was, as Nargott did in this case.
Added 18 minutes later
In general, I think that reducing speed or increasing the arena size will benefit tactical "shoot-and-defend" and "magic-and-defend" strategies – that is, ranged units will win in this case. Then you could slightly reduce the damage of ranged units and the power and cost of direct damage spells.
