a “winded” effect that progressively lowers dex and str and/or speed as you get tired sounds cool. it could work like pain that accumulates over time
-when using large and cumbersome melee weapons (adjusted to weapon’s weight or attack speed)
-moving over rough terrain, climbing fences and swimming
-when moderately encumbered or when you go over the volume limit
-when dodging or breaking grapples
-maybe a toggle-able “sprint” button that raises speed by a percent (possibly add a trait that raises it)
wielding a big machine gun and sprinting would slow you down
this would make combat more difficult which is - in my opinion - a good thing and would make overpowered end-game melee characters a bit less viable.