If a player has a heavy backpack, launcher, heavy armored vest and other big weight, then he will only tire quickly and on this, the weight effect ends.
But until he is not too tired, the player will always have the same maximal movement speed with a soldier, that does not have heavy equipment! Unfortunately, this moment is not realistic, because We know, that in the real life, the man without the big weight, will always move faster.
It would be wonderful to see the some impact (not big!) of portable weight on maximal-speed of movement. You can also add the sound carrying equipment. Thus, the player at any point in the movement might feel the weight of their equipment.
For example, the portable weight, can have a linear effect on the movement speed with only a small slowdown, in the range of 10-12% for medium-speed movement, and 15%-30% slowdown for the maximal movement speed of infantry man.