Reducing movement speed only prolongs the game, and essentially, you'll have to rework a lot of things because it will change the races' tactics.
It's strange to say, but Nival managed to create a balance where the game progresses at a decent pace and is still interesting to play.
For example, some people like that they can throw almost their entire army at the enemy in one turn and defeat them. This creates more dynamic gameplay, but less tactical depth.
However, simply reducing the top movement speeds isn't enough, because after that, the problem of uniform movement speeds arises, as in Heroes 6 or 7.
Therefore, a small reduction in infantry movement speeds, around 20-30% (but across the entire range, including actively using a speed of 3), would be appropriate. 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 movement speed diversity can be achieved, flying units can have a distinct role, and the problem of "half the army attacking in one turn" can be solved.
