From what I understand, there is no ceiling for how fast an attribute can be tapped. Because of this, some very interesting things can happen.
Steel feruchemy stores speed. if one were able to store enough speed and tap it fast enough, they could, theoretically, go faster than the speed of light. As of yet, it is unknown what would happen if this was broken, but I know if a few theories: Time travel and leaving the universe(parallel universe)
Storing enough speed to do this would be incredibly difficult, as feruchemical attributes give diminishing returns the faster it is tapped. A solution to this would be to have a bunch of unkeyed steelminds filled by a lot of people and then used really quickly by one person
What do you people of the 17th shard think?