In TROM2 I reduced the marching speed of all land units by 20% (and increased the speed of ships by 20%) with a global multiplier. It may be possible to do this for artillery, infantry and cavalry separately, but I have to test that.
It's an interesting question how the marching speed of cavalry was compared to infantry. I am not shure if cavalry was really 50% faster then infantry over many days of campaigning. Maybe that is something for the historical research subforum.
EDIT: With some ingenious relabeling of effects it's indeed possible to change the movement speeds of unit categories globally without having to edit each single unit, which would be a chore because you have to do that for all starting units in the campaign files. I await your suggestions...
