Maybe I'm not stating my question correctly.

This is entirely a theoretical question, so imagine there are no blocks, but we still do have hashing power on the network (or stake, for a POS chain) in order to be able to reach some kind of consensus.

Under these broad assumptions, is the optimal confirmation time (fastest possible time to form a robust consensus) simply the time it takes for >50% of the hashing power/stake to agree on any given transaction, or is it more complicated than that?