1) Get any hexadecimal editor. For example, hiew.exe
2) Open heroes3.exe (h3wog.exe, h3te.exe, h3wt.exe, etc.) in it.
3) Find the following code snippet:

These are the hexadecimal values for movement point consumption on different terrains, i.e., 4Bh = 75d, 41h = 65d, 32h = 50d
Next, change all instances of 4B (75) in this section to 41 (65). This will make the movement speed on slow terrains the same as on medium terrains.
4) To make it look more consistent, you should also change the graphics for slow terrains to match medium terrains.
To do this, find the following code snippet:

And change dirtrd.def to gravrd.def
5) PROFIT!
Keep in mind that the addresses of these sections may be different in different executable files. In particular, SoD 3.2 and Platinum have different executables and, therefore, different section addresses.