Johnny Two-Finger’s base move speed is 30 ft. Johnny goes out to battle wearing a shiny set of mithral breastplate.
What is his move speed in 3.5 and Pathfinder?
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His move speed remains 30 ft. in both 3.5 and Pathfinder.
Most mithral armors are one category lighter than normal for purposes of movement and other limitations.
While there's some room for disagreement about what "other limitations" means, movement is explicitly called out. Thus, a mithral breastplate is unambiguously light armor for movement purposes, and light armor doesn't slow you down in either 3.5 or Pathfinder.