Individual Speed, 2nd Variant (5e Variant Rule)
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This rule is a much more simplistic take on individual variation in walking speed than the other two versions of the rule, without changing how ability scores work or adding an additional step to combat.
All playable or humanoid races have a base walking speed of 30 feet + their Dexterity modifier. For more drastic variation in speed, the DM may choose to replace the modifier with any bonuses the creature might apply to either the Athletics or Acrobatics skills.