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I think speed is a MUCH more dominant factor in speed to 1B and SBs than in defense.
Speed to 1B in particualar. LHs have a big advantage, and some guys like Ichiro have mechanics that give them a head start, but in general its mostly speed.
I think theres a little more skill involved in SBs, but its also mostly speed relative. The extent of skill is in judging leads and knowing "when" to go.