Careful, you're hitting the limits of the abstraction of the D&D combat system.... At some point it will simply fail to make sense.
You can move 30' and still attack or cast a spell (or equivelent action) all in one round. I guess then each action would be 3 seconds, but you still can't take two attack actions or two spellcasting actions instead of moving. Why? The rules say so, that's why.
I suppose if you only move 30', then you are taking the entire 6 seconds to do it (that's why you don't suffer AoOs - you're being careful).
You can double move, if you like, and move 60' in one round, or 10' per second.
You can run, and move 120' in a round, or 20' per second.